US2252530A - X-ray timing apparatus - Google Patents

Description

All@ 12, 1941 R. 1 swEENY ETAL 2,252,530

X-RAY T IMING APPARATUS Filed Sept. 6, 1940 4 Sheets-Sheet 1 Aug. l2, 1941.

R. L. swEENY ETAL 2,252,530

X-RAY TIMING APPARATUS Filedept. 6, 1940 4 sheets-sheet' 2 I 771/@ al y. MW

Aug. 12, 1941. R. L. swEENY ETA. l X-RAY TIMING APPARATUS Filed Sept. 6, 1940 4 Sheets-Sheet 3 Aug- 12, l941- R. L. swEENY ETAL 2,252,530

XRAY TIMING APPARATUS I Filed Sept. 6, 1940 4 Sheets-Sheet 4 Patented Aug. 12, 1941 X-RAY TIMING APPARATUS Raymond Llewellyn Sweeny, Forest Hill Village, Toronto, Ontario, and George Albert Richards, Toronto, Ontario, Canada, assignors to Fen-anti Electric Limited, Toronto, Ontario, Canada Application september 6, 1940, serial No. 355,702

16 Claims. (Cl. Z50-95) The present invention relates to apparatus for timing X-ray exposures.

In producing a satisfactory X-ray film, two principal factors determine the resultant quality of the film. The contrast is determined by the voltage used on the X-ray tube anode, and the density is determined by the total quantity of X-rays, which can be expressed in terms of the milliampere-second product. Milliamperes and time separately have no importance to lm density; it is only their product that has significance in this respect. ,Y

The comlmon method of securing acertain desired total number of milliampere-seconds` has been to set the filament of the X-ray tube to a temperature which will produce a value of milliamperes anode current which, when multiplied by the time set on the timer, will produce the desired product of milliampere-seconds. This system has many disadvantages. tube filament must be set accurately, usuallynecessitating one or more calibrating exposures, which may waste valuable iilm and which reduces the useful life of an expensive tube. inaccuracies in setting the X-ray timer and milliamperage the use of the presentinvention, to determine ilm density with one setting instead of two, but any fluctuations in line voltage, which would be magnified through the exponentiall filament characteristic to the anode current, are accurately compensated fo-r. This ensures the production of a properly exposed lilrn in all cases. Extreme accuracy of filament setting is not required, and calibrating exposures are thereby eliminated with corresponding increase in the life of the X-ray tube.

Despite the fact that time is an important factor in some cases, notably in chest work, this does not reduce the usefulness of applicants milliampere-second timer, since a simple calculation will enable the proper time and milliamperage to be determined.

Sin-ce the product of milliamperes and seconds determines density, a calculation is implied in every case when the customary straight timer is used. Applicants milliampere-second timer only requires this calculation occasionally and the dominant factor is always set with the subordinate factor calculated. The straight timer requires the setting of two subordinate factors to secure the desire-d dominant factor.

The X-ray Attempts have been made to provide a milliampere-seconds timer but so far as applicants are aware no practical and accurate timer of this kind has hitherto been proposed.

The prime object of the present invention is to provide an accurate and relatively simple apparatus for use with an X-ray transformer unit, which will provide such milliampere-seconds timing with great accuracy and uniformity of operation.

A further object of the invention is to provide means' for compensating for the non-linearity of the (exponential) charging curve of the condenser-resistance timing circuit so as to obtain the same operating characteristic as would be obtained with a linear charging curve, thereby making the device accurate over its entire range of operation.

A further object of the invention is to provide means for utilizing a very large portion of the `charging curve of the condenser-resistance timing circuit so as to allow the use of a condenserresistance timing circuit having a small timeconstant in such a manner that the voltage appearing across the condenser is relatively large, thereby making `for stable operation of the device. l

A still further object of the invention is to provide .means to allow the use of the heavy switchgear necessary to control the large primary currents whichare used in high-power X-ray apparatus, and to provide means for compensating for the time lag inherent in the operation of such large heavy switchgear.

A still further object of the invention is to provide an accurate apparatus for use with an X-ray transformer unit, which apparatus will allow exposures to be timed in terms of milliampere-seconds in such a mianner that the percentage error in setting the scale will be constant, rather than the amount of error asis the common practice.

A still further object of the invention is to provide means whereby the `apparatus will not operate until the filaments of all electron tubes therein shall have come up to operating temperature, thereby eliminating the possibility of operation under conditions which might produce inaccurate results.

According to applicants invention the actual X-ray tube anode current is utilized to actuate the timing device. The property whereby a condenser needs a certain definite time to acquire a certain definite charge is utilized to form the timing element. Various methods to make use of this property are hereinafter described.

The invention consists in the combination and arrangements of elements hereinafter described and more particularly pointed out in the appended claims.

Referring now to the accompanying drawings which illustrate diagrammatically, and by way of example only, various embodiments of the invention:

Figure 1 is a diagram illustrating the principle of the invention,

Figure 2 is a diagram similar to Figure l but showing an arrangement in which the use of batteries is eliminated,

Figures 3 and 4 are diagrams of detail modiiications,

Figure 5 is a diagram of a preferred form of the invention for use with a half-wave X-ray machine.

Figure 6 is a diagram of a preferred form of the invention for use with a full-wave X-ray machine.

In Figure 1 the numeral I indicates the alternating current supply which is connected to the primary 2 of the X-ray transformer T through a switch 3 of the X-ray contactor 4 which includes a coil 5. The secondary 6 of the transformer T is connected at one end to the cathode 'I of the X- ray tube 8 and at the other end to the anode 9 and to ground through a resistance R1. 'I'he top of .the resistance R1 is connected to the grid of a control tube I through resistances R2 and R3, and the lower end of the resistance R1 is connected to the cathode of the tube I0 through a grid bias II. A timing condenser C is connected at one side to a point between the resistances R2 and R3 and at the other side to the anode of the X-ray tube 8. A switch I2 is connected in parallel with the condenser C and preferably a condenser C1 is bridged across the connections to the grid and cathode of the control tube IB. The plate battery, shown at I3, is connected to the plate of the tube III through one side of a switch I4 and the coil of a relay I5.

One of the supply lines I can be connected to the other through the other side of the switch I4, the movable element of the relay I and the coil 5 of the contactor 4.

Resistance Rz is preferably large with respect to resistance R1. The control tube I0 may be of either hot cathode or cold cathode type with an appropriate type of xed bias of such value that with switch I2 closed, the tube I0 is in its nonconducting phase. Resistance R3 is used to prevent the ow of excessive grid current when the tube I0 is conducting.

To obtain an X-ray exposure it is necessary either rst to open switch I2 and then close switch I4 or to perform these two operations simultaneously, preferably using a suitable switch or relay as will be later described.

On closing switch I4, the coil 5 of the contactor 4 is energized to close the contactor switch 3 so that the exposure is thus initiated. Simultaneously, the closure of switch I4 also applies plate voltage to the tube IU through the coil of relay I5. Thus far the tube I0 is non-conducting.

The X-ray tube current now flows through resistance R1 in such a direction as to cause a voltage drop Ex of the polarity shown. Ex appears The voltage Ee, across condenser C, increases exponentially with time, and appears as a component of the total grid bias on the tube Il), acting in opposition to the xed bias I I.

Instead of applying voltage Ec directly in the bias circuit of the tube I0 it can be applied to the grid of a D. C. amplier of appropriate design, not shown, so that the output of this amplier then acts in the grid circuit of the tube I0. Thus as voltage Ec increases, the total resultant bias decreases, and on reaching a predetermined value, the bias becomes insufcient to prevent the flow of anode current, and so the tube II) begins to pass plate current. This plate current energizes the coil of the normally-closed relay I5, causing it to open, thereby automatically opening the circuit of the contactor coil 5, so that the switch element 3 opens and thus terminates the exposure. At the end of the exposure switch I4 is opened and switch l2 is closed and the closing of the latter switch discharges the timing condenser C in preparation for the next exposure.

With the arrangement just described, the condenser C1 in the grid circuit of the tube I0 was found to be necessary to prevent surge impulses produced in closing switch I4 from reaching the grid.

The value of milliampere-seconds to be obtained can be determined by an adjustment of resistance R1 or R2 or condenser C, any one of which will aiect the time required for the charge voltage E@ to build up across the timing condenser C to the required amount to cause the tube I 0 to pass plate current.

Other methods of determining Values of milliampere-seconds to be obtained are varying the grid bias voltage, anode voltage, or even the cathode heating current (in the case of a hot cathode control tube), thus changing the amount of charging voltage E@ required to operate the tube I0. These methods, however, are not as satisfactory as those described in the preceding paragraph.

A triode, tetrode, or other multi-element vacuum tube may be used in place of a gaseous discharge, grid controlled tube as the control tube I0. With the high vacuum type of control tube the action is slightly different. In that case the plate current builds up simultaneously with the condenser charge voltage Ec until it reaches such a value that it will be suiiicient to cause the normally closed relay I5 to open. In such an arrangement it is found feasible to determine the milliampere-seconds value by adjusting the spring tension of the relay I5, as well as by any of the previously described methods.

Another method of using the vacuum type electronic tube is to allow this tube to be conducting during the exposure, operating a normally-open relay (or even operating the X-ray contactor 4 directly). In this case voltage Ec is added to the iixed bias in such a manner that the two are additive, so that as E@ increases, the vacuum type control tube current falls, finally reaching a value low enough to allow the relay or contactor to open.

For accurate operation of the circuit shown in Figure 1, it is assumed that the condenser voltage Ec increases in a linear relationship with time, assuming the voltage EX is constant. Actually this increase is exponential, but if the operation of the circuit in Figure 1 is confined to a small portion of the beginning of the curve, then the increase is almost linear and reasonably accurate operation is obtained. In order to secure substantially linear operationV by utilizingr this small initial portion of the curve, itl isY necessary to make the time constant of resistance R2 multiplied by the capacity of condenser C quite large.

This product (RZXC) should be numerically about 3 times as great as the desired longest ex-v posure, measuredl in seconds. For instance, suppose it is desired to time a maximum exposure of 20 seconds, then RZXC should be:

If we let :10 microfarads, then-.R2 must be:-

=6,000,000 ohms or 6 ncegohms` Where tztime in` seconds xzX-ray tube current. in milliarnperes K=arbitrary constant.

Since IX isin milliamperes andi t in seconds, this means that under the above conditions the value of milliampere-seconds obtained is consta-nt, regardless of the magnitude of the X-ray tube current Ix.

By changing any of the circuit conditions, as previously described) it is possible' to secure any desired'value of milliampere-seconds.

In practice, it is desirablev toeliminateI batteries as a sourceof- Voltage supply, and to design the device for operation froml any convenient alternating current supply service. An alternating current form of the circuit shown in Figure 1 is illustrated in Figure 2 in which the corresponding elements are indicated by the sa-me numerals as in Fig-ure l as` far as possible. The supply leads lA are connectedV toY the' primary 2 of a transformer T`, and the filament of the tube Il) is connected toy a secondary coil Ea. A secondary coilA 6b' is connected to the filament cfa full- Wave rectifier tube- IB" the plates of whichL are connected to the ends of a secondary coil 5C. The middle point of the secondary 6a is connected to an adjustable tapv at I'l to a resistance t8 which is connected to the centre points of secondaries (5b4 and 5c in parallel with a filter unit consisting of a choke coil L and condensers C3 and C4. The ends of the resistance IB`` are also connected respectively tothe grid", and to the plate of the tube Ii)v through the switch M and the coil of the relay i5.

In this particular arrangement, bias is obtained by making the cathode of the controlr tube IU positive (rather than the grid negative) with respect tothe common lead. Alternating current heating current is" supplied to the filament and its electrical centre is obtained by means of the centrer-tapped winding 6a or if desired by'means of av centre-tapped resistor across the windingV as is common practice.

D. C. for the anodeandy grid of control tube t0" is obtained from the output` of the filtered fullwave rectifierY system I6, C3, C4, L, T.

As previously stated, the curve of condenser chargel voltage against time `is not the ideal straight linegbut an exponential curve. By us ing aI combination of resistance R2 and capacity C with alarge time-constant, a straight line isr quite closely approximated, but this arrangement has the disadvantage? that the valve of Ec necessary to cause the control tube I0 toi pass plate current is necessarily small, so longas the range of EX is kept down within reasonable limits. Since such a small variation in gridV bia-s is depended` on to cause the control tube It to'function, operation becomes somewhat critical, and. quite sensitive to changes in tubeA characteristics, line voltage fluctuation, and theV like, By de-y creasing theA time-constant of the--RzXC combination it is therefore possible toimprove the sta-- bility of operation',l but at the sacriiice of true linearity' of operation. With aY small time-constant combination the ratio of` long to short exposures for any given milliampere-seconds set ting is;` tooA great.

To overcome this, a circuit element may be added, in the form of a small iixed voltage' supe ply inseries with This additional voltage is supplied forexample', from a tap at 2l]L on the same D. C. supply (see resistance i8, Figure 2)v asis used for the plate and grid circuits of theq control tube I, and is so connected that it adds to` the voltage EX as shown in. Figure 3.

This E. M. F., E1 being fixed, will' contribute an increasing proportion of the condenser charging voltageV as Ex decreases. E1 is only a fraction ofY one percent of the maximum value of Ex (that is` during. the shortest exposure) but amounts to possibly fty percent of the minimum value of EX (that is during the longest exposure)'. Thus Er acts toV shorten the long exposures, and` has a negligible effect on the very short exposures.

By properly adjusting E1 it is thus possible to obtain almost perfect linearity over a whole range ofv exposures up to about 1.3 time-constants, for instance, with C equalling IO'micro-farads and R2V equalling 6 megohms, it would be possible to time up to' about l.3 60'=78 seconds.

One feature that materially limits the sphere of accurate operation of any true milliampereseconds timer is the lengthening ot the very short exposures due to the slow action of most contactors in opening the circuit. With a milliampere-seconds timer the timing action does not begin until the contactor is closed, so that the time required for closure is of no consequence. However, at the end of the exposure,A when the timer cuts 01T the contacter coil current, the' X-rayV current continues to flow until the contactor has opened. With the timer providing anv exposure of 1/25 second, and a contacter lag of j/sc second, say, an overall exposure time of 3/50 second is Obtained, or a-n error of 50%.

With the circuit rnodiiication` shown in Figure 4 the lengths of all exposures provided by the' timer can be shortened, by equal amounts.`

This lag compensator circuit consists of a resistor R4 and a condenser C2 connected in parallel between the resistance R2 and timing condenser C. With this arrangement, the E. M. F. appearling at the grid of the control tube IU is the sum of the xed bias, the condenser charge volt`- age, and the IR drop in resistance R4. The rate' of increase of En is directly proportional to the'v current through resistance R4, and hence, tothe IR drop across resistance R4. Also, during any given exposure, a given interval of time, say 1go second, is represented and accompanied by a certain increase in the Value of Ec. The amount of this increase is directly proportional to the IR drop in resistance R4. Thus, when this latter IR drop is added in series with Ec, the length of all exposures will be decreased by a constant amount. By adjusting resistance R4 all exposures can be shortened by an amount equal to the contactor opening lag, and thus compensation can be obtained for this lag. The condenser C2, is included to lter out any high peaks which may occur in the distorted X-ray current wave form. Without this condenser operation may be somewhat unstable.

A preferred form of the improved timer, for use in conjunction with a half-wave X-ray machine, is shown in Figure 5, including all the circuit features thus far discussed.

In this arrangement, resistance R1 is used as the milliampere-seconds control. Resistance R2 is arranged in a suitable number of sections and is used as a means of changing the range of operation which is set by means of a range switch i 2I. In place of switches I2 and I4 shown in Figures 1 and 2, the desired operation is obtained by using a push-button or foot-switch 22 and a second relay 23. The relay 23 comprises a coil 24 shunted by a condenser C5 and connected to opposite ends of the resistance I8 through the push-button 22. The relay 23 also comprises a pair of blades 25 and 26, the former being normally closed and coacting with a contact connected to the grid of the tube IQ and the latter being normally open and coacting with a contact connected to the movable element of relay I5.

The secondary of the X-ray transformer T2 is divided into two parts 2! and 28 one of which is connected to the filament 'I' and to the resistance R1 through a direct current milliammeter 2S, while the other is connected to the anode 9 and to ground.

The primary 2 of transformer T2 is connected across the A. C. source I through the normally open contactor relay 4, the source being provided with a switch 36. The source I is also connected to the primary of a transformer T1, the secondary of which comprises three windings 3|, 32 and 33. spectively to the middle points of secondaries 32 and 33 through a lter consisting of a choke coil L and condensers C3 and C4. The ends of winding 32 are connected to the lament, and the ends of winding 33 to the plates of a rectifier tube 35. The ends of winding 3| are connected to the lament of control tube II) and the middle point of winding 3I leads to a tap 36 on resistance I8. The lower end `of resistance R1 leads to a tap 3l on the resistance I8, to supply the fixed voltage E1. (See Figure 3.)

'Ihe following is a list of suitable values for the various elements in Figure 5, though the invention of course, is not limited to such values:

R11,000 ohms variable (3l-.001 microfarad Rg--Range l-l00,000 ohms Cz-LO microfarad Range 2500,000 ohms Range 3-2.5 megohms Ita-400,000 ohms R4-5,000 ohms variable 18 5,000 ohms total C-4.0 microi'arads C14-4.0 microfarads (l5-0.25 microfarad C 10.0 mierofarads' L- hcnries By way of example the voltage drop across winding 3| may be 2.5 volts, across winding 32, 5 volts, and across each half of Winding 33, 125 volts. Tube I0 may conveniently be of type 2A4G and tube 35 of type 82.

The ends of resistance IB are connected ref The operation of this form of the invention is as follows:

The line switch 39 is closed to connect the whole apparatus to the A. C. line. The transformer T1 is now energized and so supplies filament heating current to the control ltube I!) and the rectifier tube 35 in the timer. Transformer T1 also supplies A. C. to the plates of the rectifier tube 35, and the latter converts the A. C. to pulsating D. C. The pulsating D. C. is ltered by unit L, C3, C4, thus giving substantially pure D. C. across C4 and I8, having the polarity indicated. This pure D. C. is required to supply the plate and grid circuits of the control tube I0.

It Will be noticed that with relay 23 in the normal position shown, the grid of control tube I0 is connected through resistance R3 to the negative side of the D. C. supply, whereas the lament of control tube I8 is connected, through the transformer winding 3l, to the tap 36 on resistance I8, thus making the lament positive with respect to the grid. In this way a fixed grid bias is obtained.

As yet no anode current flows in the X-ray tube 8, since transformer T2 is not yet energized. We do assume, however, that the filament of the X-ray tube is appropriately heated at this point.

Next the exposure push-button 22 is pressed to connect the D. C. supply directly to the coil 24 of relay 23 and at the same time to the plate of the control tube I 0 through the coil of relay I5. The fixed grid bias on the control tube is greater than the critical bias at this point, and so no plate current flows as yet. This closure of button 22 operates relay 23 and energizes the coil 5 of the heavy primary contactor 4 to cause closure of the normally-open contact 3. 'Ihe normally closed contact of relay 23 has, until now, acted as a short circuit across the combination of C, R4 and C2. This short-circuit is now opened. To simplify explanation, the lag compensator circuit which consists of R4 and C2 may, for the time being, be regarded as short circuited.

The contactor 3, 4, 5 is a heavy device with large contacts usually immersed in oil to carry the heavy primary current of the X-ray transformer. Although similar in diagrammatic representation, it must not be confused with the very much smaller and lighter relays I5 and 23, which are parts of the timer itself.

On the closure of the primary contactor 3 the X-ray transformer T2 is energized and so X-ray anode current is caused to flow.

Since current is now flowing in resistancce R1 there is a voltage drop across it proportional to the current flowing and having the polarity indicated. To this voltage (Ex in Figure 3) is added the voltage (Er in Figure 3) in the small portion of resistance I8 between the lower tap 31 and ground, which drop is caused by the X-ray anode current plus the current flowing in resistance I8 from the rectier tube 35.

Ex plu-s E1 now appears across the timing circuit (consisting of resistance R2 and condenser C in series) and the condenser C begins to charge up at a rate depending on the values of R2 and C, and the voltage EX plus Er. I'he charge Voltage across C is of the polarity shown and so acts in opposition to the fixed bias voltage.

When the grid bias on the control tube II) is decreased by the predetermined amount the point of critical bias is reached, the control tube ignites, and the plate current in the control tube I Il flows and energizes the coil of relay I5. The

normally-closed contacts of this relay now open, thus de-energizing the coil of the primary contactor 4, thereby opening the primary circuit of the X-ray transformer T1, and ending the exposure.

In the ideal arrangement the exposure would end at the instant when condenser C' became charged up to the point of critical bias, whereas it will be seen that there are three steps that follow this, namely, ignition of tube I0, opening of relay I and opening of contactor 3. The first can be considered to take zero time, but the opening of relay I5 and especially the opening of contactor 3 take measurable periods and actually add to the exposure time. For this reason the lag compensator circuit, comprising resistance R4 and condenser C2 was devised, and the operation of this circuit has already been described with reference to Figure 4.

The lower tap 31 on resistance I8 is for the curve compensator, which has been described with reference to Figure 3.

After the exposure is completed the push-button 22 is released. This de-energizes the relays I5 and 23 so that they return to their normal positions shown in Figure 5. The normallyclosed contact of relay 23 is used to short-circuit timing condenser C to remove its charge, in preparation for a subsequent exposure.

In the full-wave circuit shown in Figure 6 the secondary of transformer T2 is connected to the X-ray tube through a bridge rectifier circuit indicated at 38.

Transformer winding 21 is connected to resistance R1 through an A. C. milliammeter 23. The lower end of resistance R1 is tapped to curve compensator resistance 39 which is grounded at one end and connected at the other end to ground through a secondary winding 40 of transformer T1.

The top of resistance R1 and the bottom of resistance 39 are each connected to sets of resistances R5 and Re provided with range selector switches 4I, which are connected as indicated to double diode tubes 42, which are connected on one side to the bottom of condenser C and to the normally-closed contact 25 of relay 23, and on the .other side to the lower end of resistance I8.

The top of the lag compensator circuit R4, C2 is connected to a tap 43 near the bottom of resistance I8 for cancellation of Contact E. M. F. developed in the double diode rectifier tubes.

The relay coil 24 is connected to the bottom of resistance I8 through a slow heating rectifier tube An electrostatic shield may be provided in transformer T1 as indicated at 45.

The following suitable values of the elements of Figure 6 are given by way of example only:

SR1-1,000 ohms variable (Jr-.001 microfarad 'Rn-100,000 ohms Cz-LO microfarad lil-5,000 ohms variable (la-4.0 microiarads Rs-Range 1-50,000 ohms each (3l-4.0 microfarads and Range 2-250,000 ohms each Rrr-Range 3-1.25 megohms each C15-0.25 microfarad 18 5,000 ohms total C -l0.0 microfarads L =30 hennes exposure before this heating period has elapsed. This is done by providing, in the coil circuit of relay 23 the slow-heating rectifier tube 44.

One more point of difference between the two circuits is in the curve-compensator arrangement. In the half-wave form shown in Figure 5 a tap 31 on resistance I3 was used to supply D. C. which was added to the D. C. drop in resistance R1. In the full-wave timer, since there is A. C. flowing in resistance I3, it is added to an A. C. voltage obtained from the additional winding 43 on transformer T1.

For simplicity, in Figures 5 and 6 the primaries -of both transformers T1 and T2 are both shown as being connected to al single A. C. source and this arrangement can be used in practice when desired, but a common arrangement is to connect the two transformers to separate sources, for example transformers T1 to a supply -at volts and transformer T2 to a supply at 220 Volts.

It will be understood that the improved timer may be used in conjunction with any type of X-ray machine, whether the X-ray filament heating is adjusted by a continuously adjustable resistance or choke or by tapped resistances or otherwise.

It will also be understood that the lag compensating means will be required in the -timer only when a contactor with relatively slow-moving parts is employed in the X-ray machine. Certain X-ray machines employ an electronic relay system in place of a mechanical contactor, and in such cases lag compensating devices are unnecessary.

The foregoing description and accompanying drawings are given by way of example only and any modifications within the scope of the appended claims may be resorted to without departing from the spirit of the invention.

What we claim is:

1. Apparatus for timing X-ray exposures cornprising a low resistance to be connected in the secondary circuit of the X-ray tube high-tension transformer, a timing circuit including a relatively high resistance and a condenser in series and connected in parallel with said' low resistance, a control tube having a voltage-sensitive element connected to said condenser so as to respond to changes in the voltage across said condenser, switching and relay means connected with said control tube and with the primary winding of the X-ray high-tension transformer to cause automatic opening of said primary winding when said condenser has attained a predetermined charge, and means for compensating for the non-linear charge curve of the timing condenser.

2. Apparatus as claimed in claim 1, wherein switching means are provided for short-circuiting said timing circuit after each exposure.

3. Apparatus as claimed in claim l, wherein switching means are provided for automatically short-circuiting said timing circuit after each exposure.

4. Apparatus as claimed in claim 1, wherein a second high resistance is inserted between said timing condenser and the voltage-sensitive element -of the control tube.

5. Apparatus as claimed in claim 1, wherein a second high resistance is inserted between said timing condenser and the voltage-sensitive element of the control tube, the timing condenser abeing bridged by a relatively low capacity condenser at a point between said second high resistance and the voltage-sensitive element of the control tube.

6. Apparatus as claimed in claim 1, wherein said timing circuit includes also a rectifier connected between the relatively high resistance and timing condenser so that a unidirectional voltage is applied to said timing condenser.

7. Apparatus as claimed in claim 1, wherein means are provided for compensatingT for time lag caused by delay in the opening of relatively slow-moving parts of the circuit.

8. Apparatus as claimed in claim 1, wherein means are provided for compensating for time lag caused by delay in the opening of relatively slow-moving parts of the circuit, said means comprising a resistance and a condenser connected in parallel with each other and in series with the timing condenser.

9. Apparatus as claimed in claim 1. wherein said means compensating for the non-linear charge curve of the timing condenser, consist of means for adding a suitable fixed voltage to the voltage app-lied to the timing circuit by the voltage drop across the low resistance.

10. Apparatus as claimed in claim 1 wherein the high resistance in the timing circuit is arranged in a plurality of parallel sections cooperating with a range selector switch.

11. Apparatus as claimed in claim 1, comprising also -a low-tension transformer, a resistance connected with secondary windings of said transformer, a rectifier between said windings and said resistance, a lter unit between said rectifier unit and said resistance, a normally open relay connected across said resistance through an exposure switch and a normally closed relay having a switch connected to said normally open relay and a coil connected to the plate of the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit. of the high-tension transformer of the X-ray tube.

12. Apparatus as claimed in claim 1, comprising also a low-tension transformer, a resistance connected with secondary windings of saidtransformer, a rectifier between said windings; and said resistance, a filter unit between said rectifier u nit and said resistance, `a normally open relay connected across said resistance through an exposure switch and a normally closed relay having Ya switch connected to sai-d normally open relay and a coil connected to 'the plate of .the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit of the high-tension transformer of the X-ray tube said normally open relay also comprising a normally closed switch arranged to short-circuit the timing circuit after each exposure.

13. Apparatus as claimed in claim 1, comprising also a low-tension transformer, a resistance connected with secondary windings of said transformer, a rectifier between said windings and said resistance, a filter unit between said rectifier unit vand said resistance, a normally open relay connected across said resistance through an exposure switch and a normally closed relay having a switch connected to said normally open relay and a coil connected to the plate of the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit of the high-tension transformer of the X-ray tube the low resistance connected in the secondary circuit of the high-tension transformer being connected to a point adjacent the negative end of the resistance connected with the low-tension transformer.

14. Apparatus as claimed in claim 1, comprising `also a low-tension transformer, a resistance connected with secondary windings of said .transformer, a rectifier between said windings and said resistance, a filter unit between said rectier unit and said resistance, a normally open relay connected across said resistance through an exposure switch and a normally closed relay having a switch connected to said normally open relay and a coil connected to the plate of the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit of the high-tension transformer of the X-ray tube the ends of a secondary winding of the low-tension transformer being connected to the cathode of the control tube and the centre point of said winding being connected to the resistance connected with the low-tension transformer.

15. Apparatus as claimed in claim 1, comprising also a low-tension transformer, a resistance connected with secondary windings of said `transformer, a rectifier between said windings and said resistance, a lter unit between said rectier unit and said resistance, a normally open relay connected across said resistance through an exposure switch and a normally closed relay having a switch connected to said normally open relay and a coil connected to the plate of the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit of the high-tension transformer of the X-ray tube a slow-heating rectifier tube being interposed between the coil of said normally-open relay and the resistance connected with said low-tension transformer.

16. Apparatus as claimed in claim l, comprising also a low-tension transformer, a resistance connected with secondary windings of said transformer, a rectiiier between said windings and said resistance, a filter unit between said rectifier unit and said resistance, a normally open relay connected across said resistance through an exposure switch and a normally closed relay havinga switch connected to said normally open relay and a coil connected to the plate of the control tube and to said exposure switch, said normally closed relay being also adapted to be connected to a normally open relay in the primary circuit of the high-tension transformer of the X-ray tube electrostatic shielding being providedvin the low-tension transformer.

RAYMOND LLEWELLYN SWEENY. GEORGE ALBERT RICHARDS.

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