US2136375A

US2136375A - Method of and means for magnetic inspection

Info

Publication number: US2136375A
Application number: US97415A
Authority: US
Inventors: Forest Alfred V De
Original assignee: Magnaflux Corp
Current assignee: Magnaflux Corp
Priority date: 1936-08-22
Filing date: 1936-08-22
Publication date: 1938-11-15
Anticipated expiration: 1955-11-15

Description

Nov. 15, 1938. v, 5 FOREST 2,136,375

METHOD OF AND MEANS FOR MAGNETIC INSPECTION Filed Aug. 22, 1956 2 Sheets-Sheet 1 INVENTOR 41mm Mia/0x55 T g BY - W, ATTORNEY Nov. 15, 1938. A. v. DE FOREST 2,136,375

METHOD OF AND MEANS FOR MAGNETIC INSPECTION Filed Aug. 22, 1936 2 Sheets-Sheet 2 R BY 1 W, ATTORNEY Patented Nov. 15,1938

PATENT OFFICE METHOD OF AND MEANS FOR MAGNETIC INSPECTION Alfred V. dc Forest, Cambridge, Mass, assignor to Magnailux Corporation, Chicago, 11]., a corporation of Delaware Application August 22, 1938, Serial No. 97,415

32 Claims. (Cl. 175-183) The present invention relates to improvements in methods of and means for inspecting magnetizable materials in which finely divided ferro-magnetic particles are employed to reveal the presence of minute cracks or other defects. These methods involve the creation of a magnetic field at the edges of the crack or other detect which may be accomplished by placing the specimen under test on thepoles of a magnetic yoke or electro-magnet, the latter being energized by direct current. This method is disclosed in Patent No. 1,426,384, issued to William E. Hoke, August 22, 1922.

It is also possible to create the desired magnetic field by passing a heavy current through the specimen which induces a magnetic field at right angles to the direction of current flow. The current may be either direct or alternating as described in my Patent No. 1,960,898, issued May 29, 1934.

In carrying out the method disclosed in Patent No. 1,960,898, it is necessary to establish good electrical contact with the specimen which will carry several thousand amperes per square inch. This is customarily done by clamping the specimen in a machine built for the purpose with specially shaped copper contact blocks or with flexible copper braid between the specimen and the conductor. Even with all these precautions there is the danger of burning a test specimen at the contact point or marring a finished surface, such, for instance, as that of a bearing ball.

It is an object of the present invention to overcome this diiliculty by supplying the magnetizing current for such a short duration of time that suflicient heat will not be developed to burn the surface. At the same time the voltage is greatly increased so that contact resistance is no longer a limitation on the current flow. It has been the usual practice when inspecting such parts as crank shafts to employ about 2000 amperes D. C. or- 1000 amperes, 60 cycle A. 0., per square inch of the sectional area under test, at a potential of between 4 and 16 volts, depending upon the equipment. In contrast to this method, the present method consists in carrying a current through the part of roughly the same value as before, that is, about 1000 to 2000 amperes, but for only a minute part of a second, say, one ten thousandth or one one-hundred thousandth part of a second, or even less. At the same time the potential may be increased to between 200 and 2000 volts or more, without diiiiculty. It is obvious that the heating eilect is directly proportional to the amount and duration of electrical current applied, while the magnetization is directly proportional to the amperage. Thus, by reducing the time factor to a minimum the heating effect is correspondingly reduced without reducing the magnetizing effect, except through the factor known as skin effect" which limits only the depth to which magnetization penetrates.

Another object of my invention is to provide a method which can be carried out by much lighter equipment than has heretofore been possible. I have found that by the use of electrical impulses of very short duration, the flow, of current and the magnetization, as well, are restricted to the extreme surface of the specimen so that the required current to magnetize the specimen is a function of its area rather than its volume. The small amount of energy necessary to provide magnetizing current for such a short time can readily be taken from a condenser. Obviously, this permits of the use of lighter equipment.

A further object of my inventionis to provide means for producing a high frequency oscillation and suppressing all but single waves at predetermined time intervals so that only isolated impulses are utilized.

A further object of the invention is to provide means whereby a magnetic field may be swept over a surface in such direction as to cause progressive magnetization thereof, each impulse of current producing magnetization in a different zone of the surface.

Other objects and advantages of my invention will appear in the following description of a thereof and also of certain means for carrying out such methods.

In the accompanying drawings;

Figure 1 is a diagrammatic showing of the ordinary ,wave form of energizing current in which the negative impulses are eliminated;

Fig. 2 is a diagrammatic showing of my improved form of energizing current comprising widely spaced unidirectional impulses;

Fig. 3 shows diagrammatically a set of widely spaced impulses of alternately opposite polary;

Fig. 4 is a diagram of electrical connections illustrating an apparatus for producing spaced unidirectional impulses of the type shown in Fig. 2;

Fig. 5 is a diagram of electrical connections in an apparatus similar to that shown in Fig. 4,

'preferred method and certain modifications widely spaced revealing them.

but with a relay control of the impulse producing apparatus;

Fig. 6 is a modification of the apparatus in the snown which impulses are usiE alternating current supparatus;

.lect; cal diagram of a mechanirec: currerr energised sysproducing moving contact; rspective a testing device moving f turbine .d Q 2 cracks that r wise, of an tanecusly detects Fig. is e specimen may alt for producing nately polarity such as 13 shows netizing surro' 14 shows in p rspectiv a means for ap= plying my method to an annular speci men, such, for instance, as a ball race ring, where the desired magnetization is circular.

My method of inspection involves first the creation of a magnetic field in the specimen with the lines of force disposed transversely to the supposed direction the cracks or other defects. If the specimer composed of material that will retain magneti it may be magnetized by passing alternating current therethrough, and in order to leave residual magnetic field in the specimen it is necessary to interrupt the current after it has reached maximum value and before has passed the zero point and has started in the opposite direction to cause demagnetization; after the specimen has been magnetized fine particles of term-magnetic material may be applied thereto, such, for instance, as

described in my Patent No. 1,960,899, and the particles will cluster about the cracks, prominently if the specimen under test is of a material which will not retain magnetism, such, for instance, as wrought iron or low carbon steel, the magnetic testing powder may be applied to the specimen and then be slightly tapped or vibrated while the current is being passed through the specimen so that the particles will be assisted in moving to the points where the magnetic field is disturbed by cracks or other defects.

One of the great advantages of my method is the fact that contact resistance may be discounted by the employment of high voltage. This permits of the use of moving contacts whereby a broad area may be inspected by progressive magnetization. For instance, in testing a turbine blade which is apt to develop slight defects or cracks running crosswise oi the blade my method permits of applying contact brushes of an energizing circuit to opposite edges of the blade and moving these brushes along the entire length of the blade. The current pursues a direct course from one brush to the other across the blade but since the brushes are moved lengthwise of the blade the entire area. will eventually have been subjected to the energizing current.

. eliminating such portions of This, according to the well known laws of physics, develops a magnetic field running at right angles the direction of the current and this magnetic field intercepts the cracks that are apt to develop a turbine blade.

in order to produce progressive magnetization of the blade it is necessary to employ only unidirectional impulses. For instance, in Figure l, a sine wave, a, b, c, e, is shown in which the negative portion 0, 5, shown in broken lines, has been eliminated, leaving onl; the positive portion a, b, o. By thus rectifying the current, a series of unidirectional impulses will he passed through the specimen, progressively magnetizing the specimen.

The wave form shown in Fig. 1 r presents that of commercial alternating current of say i) cycles per second which has been merely rectified by each wave ass below the zero line a y. heat such waves, however, is high because of toe time during which current applied ,4 sec. for each impulse). In order to reduce this heating effect, provide means for converting a low cycle current into high frequency cscii ations and suppress not merely such portions of each oscillotion as pass below the zero line and also sup press all but single half wave oscillations which take place at fixed intervals. Thus, I am able to produce a wave form corresponding to that shown in Fig. 2. This wave form will have the same amplitude as that shown in Fig. l and the intervals between impulses may be the same. but the length of time the current is applied and therefore the heating effect of each impulse will be a very small fraction of that developed by the wave form shown in Fig. 1. For instance, I may convert 60 cycle frequency into 100,000 cycle frequency but utilize only one unidirectional impulse at each one-sixtieth of a second, suppressing all the intervening wave impulses. Although the heat energy developed by such relatively widely spaced impulses is slight, yet the surface magnetizing eflect will be fully as great as that of the wave form shown in Fig. 1. With such relatively occasional impulses it is possible to use a high voltage with no substantial loss at the contacts.

I show diagrammatically in Fig. 4 a preferred apparatus for producing widely spaced high frequency unidirectional impulses. In this apparatus an electronic tube i5 is employed to control the periodic discharge 01' a condenser 16. specimen under inspection is indicated at I! and is engaged by a pair of brushes I9 and 20. The brush 20 is connected by line 2| to one side of the condenser i6, while the other brush I 9 is connected by line 22 to the cathode 23 of the gas filled tube IS. The anode 24 of said tube is con nected by a line 25 to the other side of the condenser IG. Within the tube I5 are the

grids

21 and 28. The grid 21 is connected through a reslstance 29 to a variable point on a resistance 30 bridging the

lines

25 and 22. The grid 28 is connected through a. resistance iii to the line 25 and through a resistance 32 to the line 22.

The condenser i5 is arranged to be charged through a current limiting resistor 34 from a conventional direct current power supply of say 400 volts. If desired, commercial direct current may be employed, or, alternating current may be used provided the current is rectified. In Fig. 4, I show a conventional means for rectifying alternating current, such means comprising a transformer 35 and a rectifying tube 36. The supareas-rs ply of current is controlled by a switch 51 which is preferably closed only after the brushes l5 and 20 have made proper contact with the specimen 48.

In operation, when the voltage across the condenser l6 reaches a predetermined value, say 350 volts, the tube, i5 becomes conducting and permits the condenser to discharge through it and through the specimen. Since the impedance of the discharge path is low the current can rise to very high values-500 to 2,000 amperes. Whenthe condenser I 6 is completely discharged the tube l5 becomes non-conducting and allows the condenser to recharge through the resistance 34. The critical voltage at which the tube becomes conducting depends upon the bias voltages on its grids which voltages are determined by the bias resistors 30, 3| and 32. The circuit is of the well-known relaxation oscillator type and will give current impulses at a rate determined by the supply voltage, the condenser capacity, and the break-down voltage of the tube l5. It will be understood that in order to handle large currents the tube I5 must be of a special type, as most gas filled tubes, particularly those with hot cathodes will not stand such large currents as I employ in my inspection system. The current impulses are unidirectional because the tube 15 prevents the current in the discharge circuit from reversing due to the oscillatory nature of the circuit.

In practice it is desirable to prevent the brushes i 9 and 20 from passing current until after a good contact is'made with the work. To this end, a low voltage relay circuit may be used such as illustrated in Fig. 5. The relay circuit 38, which bridges the brushes I9 and 20, is energized by a battery 39. In this circuit there is a relay 40 which when energized closes a gap 4| in the line 25. However, the relay will not be energized sufficiently to close the gap until the brushes have made good contact with the work. If desired a push button or other control switch 42 may be provided in the relay circuit but in any case the operation of the impulse generating apparatus will depend upon the establishment of proper electrical contact between the brushes and the work. This will prevent the operator from receiving an accidental shock.

It is possible to make the circuit operate in cynchronism with the alternating current line voltage by applying voltage directly from the transformer to one of the grids of the tube through resistance 3i, as shown in Fig. 6. In this figure parts which correspond to those in Fig. 4 are given the same reference numeral. It will be observed, however, that the resistor 3| in Fig. 6 is not connected with the line 25, but is connected by a line 43 to the secondary coil of the transformer 35. Thus, if the line current alternates at 60 cyiles per second, the impulses passing through the specimen l8 will also occur at the rate of 60 per second, but the impulses will still be isolated and relatively widely spaced from each other.

In Fig. 7, I show a mechanical means for producing electrical impulses of short duration with direct current. In this case, a source of direct current is supplied across the liner-1.45 and 4'5. In the line 45 there is a resistance 41 and a mechanical switch 48 which connects it periodically to the brush or contact 49 hearing on one side of the specimen 50. The other line 46 extends to the opposite brush 5!. A condenser 52 is bridged across

lines

45 and 46 and between this condenser and the brush 5| there is a resistance 53. The switch 45 is mechanically rotated at suitable speed and each time theswitch makes contact there will be a discharge of current stored in the condenser 52 which will produce a momentary impulse running through the specimen 5.. Oi

dinarily, such a discharge would result in a series of rapidly damped oscillations. However, by introducing the resistance 53 the damping may be increased to such an extent as to produce substantially a single unidirectional impulse at each closure of the switch. It will be understood that there are other ways in which widely spaced impulses, such as indicated in Fig. 2, may be obtained. A

In Fig. 8, I illustrate an application of my method to the testing of a blade-like specimen 54 which is composedof a metal that will retain magnetism; The specimen is attached at one end to a shaft 55 which is connected to a conductor 55 by means of a fixed brush or electrical contact 51. A brush 58 is connected to a conductor 59 which forms the opposite pole of the impulse producing apparatus. This brush may be conveniently made of copper gauze or other suitable material which will give a broad and flexible contacting surface. In operation, the circuit, represented fragmentally by the conductors 55 and 59. is energized with periodic unidirectional impulses and the brush 58, at the same time, is moved along the edge of the b'ade 54 toward the shaft 55., The current impulses will flow from brush 58 to shaft 55 and thence to line 55, and in doing so will enter the edge of the specimen following in general the dotted lines shown in Fig. 8. In other words, the current will enter the blade at right angles to the edge and then will curve toward the shaft'55. The brush 58 will be moved gradually along the edge of the blade toward the shaft and each impulse of electric current will produce a corresponding magnetic field which will be retained in the blade 54, and the direction of this field will be substantially parallel to the edge of the blade along which the brush is moved.

Thissystem oi. testing is particularly applicable to inspection of turbine blades which are apt to develop transversecracks at the leading edge of the blade.' Such cracks would be disposed at right angles to the direction of the magnetic field created by the electric flow. The penetration of this field will be confined rather closely to the 'edge of the blade and the test is applied to the leading edge of the blade. If it be desired to test the opposite edge, as well, the contact brush 58 may also be moved along the opposite edge of the blade.

Instead of having one fixed contact and one moving brush two opposed brushes may be employed, as shown particularly in Fig. 9. In this figure, I show a pair of

contact arms

60 and 5| which are hinged together intermediate their ends, as indicated at, 62. The arms are provided with contact brushes 63 and 64 respectively, and extensions of these arms at the opposite side of the pivot 62 form handles 65 and 66, respectively. These handles are normally pressed apart by a spring 61 which causes the brushes B3 and 84 to engage opposite sides of a specimen 68 to be tested. brushes 63 and 64 through leads 58 and II. The brushes are suitably insulated from the arms 50 and 6| by blocks of insulation material indicate at H and 12, respectively.

In operation, the brushes are applied to oppo- Testing current is supplied to the themagnetic lines of force.

- specimens at a time.

. surface.

site edges of the specimen 88 which is to be tested, the

handles

65 and 88 being squeezed to spread apart the brushes, and then on releasing the handles the spring 61 exerts sufficient pressure to maintain the desired contact of the brushes with the edges of the specimen 88. The current is applied and at the same time the device is drawn lengthwise of the specimen so that successive electrical impulses pass through parallel paths transversely with respect to the longitudinal axis of the specimen. This results in the production of successive magnetic fields extending lengthwise of the specimen so that any cracks or inhomogenities, such as indicated at 13, will interrupt Magnetic testing powder is then applied to the specimen and the powder will collect at points where the magnetic lines are interrupted, indicating the defects.

If desired, the test may be applied to several For instance, in Fig. 10, I show a pair of

turbine blades

15 and 18 being simultaneously magnetized by means of a device similar to that shown in Fig. 9, but differing therefrom in having contact blocks 88a and 64a which are broad enough to bridge the pair of turbine blades.

While Figs..8 to 10 show moving contact methods applied to blade-like forms the same systems may be used to magnetize portions of large pieces of material of any form. For instance, in searching for forging cracks or flakes in sections of billets, two brushes set at a distance apart or" three to six inches may he so moved as to magnetize successively all portions of the On large shafts of machines where cracks are suspected at fillets and keyways, these locations can be explored by drawing the brush contacts over the part with the current between contacts roughly parallel to the probable direction of the crack.

Fig. 11 shows another application of my improved method in which it is desired to detect cracks that extend longitudinally, as well as transversely, of a specimen 17. In this case, contact blocks 83b and 64b are provided which are not disposed directly opposite each other but are off-set lengthwise of the specimen, so that the how of current from one contact block to the other will be oblique with respect to the longitudinal axis of the specimen. Such current will set up a magnetic field at right angles to the paths pursued by the electrical impulses and such magnetic fields when the magnetic testing powder is applied thereto, will betray the presence of all cracks, except those which are disposed parallel to the field.

In all cases where movable contacts are employed it is desirable to make good contact with the brushes before the current is turned on in the circuit, and to turn off this current before the contact is broken. This may be done b; employing the system shown in Fig. 5 and the push button 42 may be conveniently located, as, for instance, on the handle 65 of the tool shown in Fig. 9.

While it is necessary to use unidirectional impulses in the case of sliding contacts it will be evident that where the contacts are fixed, alternating current impulses may be employed, such as shown in Fig. 3. A circuit for producing such impulses of alternately opposite polarity is shown in Fig. 12. In this circuit there are two stroboscopic tubes Bi! and 8| connected in parallel and fed with direct or rectified current of suitable potential, say 400 volts. The specimen 82 under .to the brush 83 and the other side thereof is connected through tube 8| to brush 84, while one side of condenser 86 is connected to the brush 84 and the other through tube to brush 8!. Thus,-the path of discharge through the specimen will be in one direction for condenser 85 and in the opposite direction for condenser 88, as shown by the

broken lines

85a and 86a. The tube which reaches a critical potential first will fire, discharging its condenser. Since the tubes are never exactly alike they will never fire at the same time. The bias on the tubes is adjusted so that they tend to run at the same frequency. When acondenser is discharged the voltage across .it is low. This will increase the voltage drop in its resistor and will slightly reduce the voltage on the other condenser which will tend to prevent the other tube from firing until the first condenser has had a chance to partly recharge. This effect will cause the tubes to fire alternately and thus produce current pulses through the specimen, such as shown in Fig. 3.

While the method as so far described has called for the passage of the current through the speci men, it will be obvious that the same effect could be obtained by placing the specimen within a coil through which the impulses pass and thereby inducing electrical impulses in the specimen. Fig. l3,'for instance, shows a winding 90 of solenoid form suitable for longitudinal magnetization of I bar stock 9! or similar shapes by impulses of high current for short lengths of time. The winding must be properly proportioned for the rapid dis-- charge of current.

In Fig. 14, a ball race ring is shown within a winding 95 through which impulses of e1ectrical current are passed. If the impulses are of high amplitude but of relatively wide spacing and small area, such as shown in Fig. 3, the heat developed is comparatively small and yet a high magnetization is obtained. After the ball race has been magnetized, it is subjected to test with the magnetic testing powder to reveal such flaws as will show up as the powder clusters about the cracks or other defects in the specimen.

While I have shown a number of systems for producing isolated electric impulses of high intensity and potential but individually of very minute duration, I do not wish to be limited to the particular systems described. It will be understood therefore that I reserve the right to employ other means for producing such impulses and for applying such impulses to specimens under test without departing from the spirit and scope of my invention as set forth in the claims.

I claim:

1. The method of magnetizing a ferro-magnetic object for the purpose of locating a supposed defect therein, which consists in passing an impulse of electric current through the object in such direction as to induce a magnetic field disposed transversely to the supposed direction of said defect, said impulse being of high voltage and amperage but of minute duration not exceeding one one-thousandth of a second.

2. The method of magnetizing a ferro-magnetic object for the purpose of locating a supposed defect. therein, which consists in passing a unidirectional impulse of electric current through the object in such direction as to induce a magnetic viii) field-disposed-transversely to the supposed direction of said defect, said impulse being of high voltage and amperage but of minute duration not exceeding one one-thousandth of a second.

3. The method of magnetizing a ferro-magnetic object for the purpose of locating supposed defects therein, which consists in subjecting the obiect to periodic pulsations of electricity of high voltage and amperage but individually of small duration and separated by inert'intervals, and so directing said pulsations as to induce a magnetic field in the object disposed transversely to the supposed direction of said defects.

4. The method of magnetizing a ferro-magnetic object for the purpose of locating supposed defects therein, which consists in producing high frequency oscillations of electric current of high voltage and amperage, suppressing all but isolated oscillations at comparatively widely spaced intervals, passing said isolated impulses through the object in such direction as to induce a magnetic field in the object of a direction transverse to the supposed direction of said defects.

5.-The method of magnetizing a form-magnetic object for the purpose of locating supposed der-fects therein, which consists in subjecting the object to periodic unidirectional electric pulsations of high voltage and amperage but individually of small duration and separated by inert intervals, so directing the pulsations as to induce a magnetic field in the object of a direction transverse to the supposed direction of said defect, andcansing displacement of the object withrespect to the pulsations in a direction transverse to the direction of the pulsations.

6. The method of magnetizing a ferro-magnetic object for the purpose of locating supposed-defects therein, which consists in generating isolated unidirectional electric impulses of high 'voltage and amperage but individually of minute duration and separated from each other by inert periods, each inert period being many times greater than the duration of each impulse, passing said impulses through the object in such direction as to induce a magnetic field of a direction transverse to the supposed direction of said defects and causing a lateral displacement of successive impulses whereby the energizing eflect of the impulses will be spread over a broad area of the object.

7. The method of exploring a farm-magnetic object for supposed defects therein, which consists in inducing a magnetic field in the object in a direction transverse to the supposed direction of said defects by passing isolated pulsations of electric current through the object at right angles to the desired direction of said field, eachpulsation being of high voltage and amperage but of minute duration and spaced from the next pulsation by an inert interval greater than the period of duration of the pulsation, and applying finely divided 'ferro-magnetic particles to the ob- .ject, whereby the particles will cluster about any defects traversed by the magnetic field and will thereby reveal their presence.

8. The method of magnetizing an object'of ferro-magnetic material for the purpose of loeating supposed defects therein, which consists in generating isolated unidirectional impulses of high voltage and amperage butindividually of minute duration and spaced apart by comparatively long inert periods, passing-said .impulses through the object between a fixed point on the object and an-opposed point-movable along the object at an angle to a direct line connecting said ,end thereof and a contact point on said edge, and

moving the contact point along said edge.

10. The method oi magnetizing an elongated object of term-magnetic material for the purpose of locating supposed defects therein, said defects being supposedly directed transversely to the longitudinal axis'of the object, which consists in generating isolated unidirectional electric impulses of high voltage and amperage but individually ofminute duration and relatively widely spaced apart, passing said impulses through the object at right angles to the longitudinal axis thereof so as to induce a magnetic field in the ob- .iect of a direction transverse to the supposed direction of said defects.

.11. The method ofmagnetizing an elongated pose of locating supposed defects therein, suspected to extend either parallel or transversely to the longitudinal axis of the object, which consists in enerating;lsolatcdunldirectional electric im-- pulses of high voltage and amperage but individually of minute duration and relatively widely spaced apart, and passing the impulses through the object diagonally to the longitudinal axis thereof. p

12. The method of magnetizing a ferro-magnetic object for the purpose'of locating supposed defects therein, which consists in converting a low frequency alternating current into separated unidirectional impulses of the same frequency and amplitude as the waves of said current but each impulse enduring fora small fraction of the rent, passing said impulses through the object in object of ferro mag'net'ic material for the purduration of the corresponding wave of said oursuch direction as to induce a magnetic field therei in of a direction disposed transversely to the supposed direction of said defects, and simultaneously laterally displacing the line of application of the impulses.

13. An apparatus for producing isolatedelectrlcal impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising'a condenser, a charging circuit therefor, a source of current adapted to energize said circuit, a discharge circuit for the condenser including means for subjecting the object to the influence of the condenserdischarge soas to induce ;a magnetic field in the object, and control means adapted to limit discharge of the condenser to isolated impulses at predetermined intervals.

14. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the 7 detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit,'a discharge circuit for the condenser including means for subjecting the object to the influence of the condenser discharge so as to induce amagnetic field in the object, means in said circuit permitting discharge of the condenser only at predetermined periods, and means for damping oscillations of the condenser discharge.

15. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including contact terminals adapted to make electrical connection with the object to pass the condenser discharge therethrough and thereby induce a magnetic field in the object, and control means adapted to limit discharge of the condenser to isolated impulses at predetermined intervals.

16. An apparatus for producing isolated electrical impulses and passing the'same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including a pair of terminals adapted to make electrical connection with the object to pass the condenser discharge through said object, and an electronic tube interposed in the discharge circuit and arranged to permit unidirectional discharge of the condenser only at predetermined intervals.

17. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including a pair of terminals adapted to make electrical connection with the object to pass the condenser discharge through said object, and control means in the discharge circuit arranged to permit unidirectional discharge of the condenser only at predetermined intervals, at least one of the terminals being movable along the object in such direction as to spread the pulsations of condenser discharge over a broad area.

18. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including a pair 01' terminals adapted to make electrical connection with the object to pass the condenser discharge through said object, control means arranged to limit discharge of the condenser to isolated impulses at predetermined intervals, a normally open switch in the charging circuit of the condenser, and means adapted to close said switch only when a connection of predetermined low resistance has been established between the terminals and said object.

19. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including a pair of terminals adapted to make electrical connection with the object to pass the condenser discharge through said object, control means arranged to limit discharge oi the condenser to isolated impulses at predetermined intervals, a normally open relay switch in said charging circuit, a relay circuit for operating said switch, said relay circuit bridging said terminals, and a supply of low voltage current adapted to energize the relay circuit, said relay circuit being so electrically proportioned that the relay switch will not close unless a contact of low resistance has been established between the terminals and the object.

20. An apparatus for producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including a pair of terminals adapted to make electrical connection with the object to pass the condenser discharge through said object, control means arranged to limit discharge of the condenser to isolated impulses at predetermined intervals, a normally open relay switch in said charging circuit, a relay circuit for operating said switch, said relay circuit bridging said terminals, 9. supply of low voltage current adapted to energize the relay circuit, said relay circuit being so electrically proportioned that the relay switch will not close unless a contact of low resistance has been established between the terminals and the object, and a manually operable switch controlling the relay circuit.

21. An apparatus producing isolated electrical impulses and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a condenser, a supply of alternating current, means for rectifying said current and charging the condenser therewith, a discharge circuit for the condenser including a pair of terminals adapted to establish electrical connection with said object whereby the condenser will discharge through the object, an electronic tube interposed in the discharge circuit and arranged to permit unidirectional discharge of the condenser at predetermined intervals, said tube including a bias grid energized by said alternating current and controlling the discharge of the condenser whereby said discharge will have a frequency corresponding to that of said alternating current.

22. Apparatus for producing isolated electrical impulses of alternately opposite polarity and passing the same through an object to induce a magnetic field therein for the detection of suspected defects, said apparatus comprising a pair of condensers, a charging circuit therefor in which the condensers are connected in parallel, a source of direct current adapted to energize said circuit, each condenser having a discharge circuit including an electronic tube adapted to control the discharge thereof, and a pair of terminals adapted to establish electrical connection with the object, the discharge circuits of the condensers being oppositely connected to said terminals whereby the discharge of one condenser through the object will be in opposite direction to the discharge of the other condenser therethrough.

23. A tool for applying pulsating current to an elongated object to induce a magnetic field therein for the detection of suspected defects, said tool comprising a pair of levers pivoted together, a contact brush mounted at the forward extremity of each lever but electrically insulated therefrom, a spring acting on the levers to force the brushes into engagement with opposite sides or said object, and leads connecting said brushes to a source of pulsating electric current, one of the brushes being advanced with respect to the other so that the current will pass through the object diagonally with respect to the longitudinal axis of the object.

24. A method of magnetizing a ferro-magnetic object which consists in electrically interlinking the object with an electric circuit, producing high frequency oscillations of electric current of high voltage and amperage, suppressing all but isolated oscillations at comparatively widely spaced intervals, and passing said isolated impulses through the circuit so as to induce a magnetic field in said object.

25. A method of magnetizing a ferro-magnetic object which consists in electrically interlinking the object with electric circuit, producing high frequency oscillations ,of electric current of high voltage and amperage, suppressing all but isolated oscillations at comparatively widely spaced intervals, passing said isolated impulses through the circuit so as to induce a magnetic field in said object, and displacing the object in such direction as to spread the magnetizing effect of the impulses over a broad area of the object.

26. An apparatus for magnetizing an object, said apparatus comprising a condenser, a charging circuit therefor, a source of current adapted to energize said circuit, a discharge circuit for the condenser including means for subjecting the object to the influence of the condenser discharge so as to induce a magnetic field in the object, and control means adapted to limit the discharge of the condenser to isolated impulses at predetermined intervals.

27. An apparatus for magnetizing an object, said object comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including means for subjecting the object to the influence of the condenser discharge so as to induce a magnetic field in the object, means in said circuit permitting discharge of the condenser only at predetermined periods, and means for damping oscillations of the condenser discharge.

28. An apparatus for magnetizing an object, said apparatus comprising a condenser, a charging circuit therefor, a source of current adapted to energize said circuit, a discharge circuit for the condenser including a portion electrically interlinked with the object so as to induce a magnetic field in the object, and control means adapted to limit discharge of the condenser to isolated impulses at predetermined intervals.

29. An apparatus for magnetizing an. object, said apparatus comprising a condenser, a charging circuit therefor, a source of current adapted to energize said circuit, a discharge circuit for the condenser including a coil surrounding said object so that the condenser discharge will induce a magnetic field in the object, means permitting discharge of the condenser only at predetermined intervals, and means for damping oscillations of thecondenser discharge.

30. An apparatus for magnetizing an object, said apparatus comprising a condenser, a charging circuit therefor, a source of direct current adapted to energize said circuit, a discharge circuit for the condenser including means for subjecting the object to the influence of the condenser discharge so as to induce a magnetic field in the object, and an electronic tube interposed in the discharge circuit and arranged to permit uni-directional discharge of the condenser only at predetermined intervals.

31. The method of locating defects in a ferromagnetic object which comprises passing periodic pulsations of magnetic flux through the object across the supposed defect and in a direc-' make electrical connection with the object to pass the condenser discharge through said object, control means arranged to limit discharge of the condenser to isolated impulses at predetermined intervals, and means adapted to close the charging circuit of the condenser only when an electrical connection is established between said terminals.

ALFRED V. nu FORES'I