US2479309A - Magnetic stabilizer - Google Patents

Description

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MAGNETIC STABILIZER Filed June 28, 1947 4 Sheets-Sheet 4 Wax/roe 6 CQJXizBDOM/Jb Gave Patented Aug. 16, 1949 MAGNETIC STABILIZER Genille Cave-Browne-Cave, Victoria, British Columbia, Canada Application June 28, 1947, Serial No. 757,817 In Canada June 4, 1946 11 Claims. (Cl. 250-415) The present invention relates to devices for use in spectroscopy and spectrochemical analysis.

The purpose of this invention is to provide magnetic stabilizing apparatus that will prevent or eliminate the sideways wandering of a light source, such as the direct current arc, used in spectrochemical analysis or in spectroscopy generally.

In the following description and claims the word arc shall mean any type of electrical source using continuous or discontinuous direct or alternating current. The word arc includes the commonly used direct current arc.

By spectrochemical, that is spectrographic, analysis is meant here the use of a spectrograph or spectroscope to analyse samples or solids or so lutions, the latter usually being dried on a carbon cup. These samples are caused to emit characteristic groups of wave-lengths of light, which light is separated into its component wave-lengths by the spectrograph or spectroscope and then the intensity or integrated intensity of the light of each wave length is either measured directly with a photo-electric device, or is photographed, and the density of the spectrum lines either measured with a suitable microphotometer, or estimated visually. Various electrical sources are used to cause the sample to emit its characteristic group of wave-lengths. .A common source is the direct current arc, between either carbon, metal, or briquetted electrodes. The present invention can be used on any type of arc.

Briefly the magnetic stabilizer of the present invention consists of the following arrangement of parts: Light-sensitive means comprising one or more pairs of phototubes in a, housing are placed at or near the two vertical edges of an enlarged image of the arc. This image is obtained by reflecting light from the arc, by means of light-reflecting means such as an aluminized mirror with a hole in it, and using a convex lens to bring the reflected light to a focus at the phototube housing. Each phototube is connected through a direct-coupled amplifier to arc-stabilizing means such as a coil of an electromagnet placed behind the arc with its magnetic axis coincident with the optic axis of the spectrograph. There are thus two amplifiers, and two coils for a direct current are and four coils for an alternating current arc, wound on a common iron core. Any sideways wandering oi the arc illuminates the phototubes unequally, thereby producing a magnetic field about the electromagnet which, acting on the arc current, that is on the the arc. In analysis of samples of powdered rock, tests for reproducibility showed that greater accuracy and precision are obtained by using the instrument of the present invention.

The invention as applied to a direct current arc will now be described in detail with reference to the accompanying drawings which illustrate the invention diagrammatically and by way of example. In the drawings:

Figure 1 is a diagrammatic representation of one particular arrangement of the various parts of the apparatus;

Figure 2 is a diagram of an example of an electric circuit connecting the phototubes of the electromagnet;

Figure 3 is a diagram of a circuit for use with an alternating current arc.

Figure 4 represents the arc and the magnetic lines of force that surround it.

Figure 5 represents the are displaced to the right and also the lines of force of the magnet.

It is generally recognized that although the direct current arc is a versatile source of excitation, it provides relatively poor reproducibility when used for quantitative spectrochemical analcharge temperature. Very recently A. T. Myers ysis. Apparently this defect is due chiefly to wandering of the cathode spot," thereby producing both wandering of the arc and fluctuations in amperage with attendant changes in the disand B. C. Brunstetter in Analytical Chemistry 19, 71 (1947) recommended a rotating magnetic field placed near the direct current arc.

The present invention deals with an instru- 36 ment which eliminates almost completely the wandering of a central horizontal slice of the arc. Almost any part could be rendered free of fluctuations, but in the present application attention is directed particularly to the part from 40 which the light is dispersed and photographed.

Normally. an arc is surrounded by a circular magnetic field, depicted by broken circles at I in Figure 1. By reasons of a well known principle, placing one pole of a horizontal bar magnet 2 near the arc 3, will cause the arc to move sideways in a direction at right angles to the magnets polar that is, magnetic, axis, The magnetic field from a north pole moves the arc to the left of the magnet, a south pole moves it to the right.

Light from the arc 3 passes through a quartz magnetic field of the arc, immediately recenters with a hole I in the centre is set between the quartz lens 4 and the spectrograph slit 8. The central part of the beam of light 9 from the properly centred arc passes through the hole in the aluminized mirror 6 thence to the dispersing system of the spectrograph, in the normal way. But light from the peripheral part of the beam is intercepted by the mirror 6 and reflected, passing through a second convex lens ID to form at plane lI-l I an enlarged image of the arc. The purpose of lens I is to bring the arc to a focus at plane lI-ll. The optical path for the beam striking plane ll--ll may be 94 cm. in length but by changing the focal length of the lens In this optical path length can be altered to suit the space available to the spectroscopist. At |ll l is a box 12 housing two phototubes I3 and I4 set horizontally as shown in Figure 1. The inner vertical side of the phototube housing l2 has two adjustable slits l5 and Is, one directly in front of each phototube. These slits are to allow light from the arc to strike the phototubes I 3 and M. The slit openings may for example be 1.2 cm. high, 0.15 cm. wide, and the two slits 4.9 cm. apart; but these dimensions will depend on the type of spectrograph used, the type of sample analysed and on the focal lengths of lenses 4 and 10. The distance between the slits is adjusted so that the visible vertical edges of the arcs image, formed on face ll-l I of the phototube housing I2 partially or completely encompass the two slit openings. It is advantageous to make each slit opening of such a height that no more than the horizontal slice of the are utilized in producing a spectrogram is used to illuminate the slits of the phototube housing.

The height of the centre of the hole I in the mirror 6 and the height of the centre of the lens ID are made so that they are in the same horizontal plane as the centre of the arc and the centre of the grating or prism of the spectrograph. The height of the phototube housing I2 is preferably adjustable. Normally it is advantageous to have it of such a height that the centre of its slits are in the same horizontal plane as the centre of the grating or prism of the spectrograph.

A bafiie I! may be installed to shield the slits of the phototube housing I2 from the direct light of the burning arc and if desired baffles (not shown) may be used as shields against the lights of the room.

Alternatively, the lens [0 may be a quartz lens and a filter may be inserted in front of the two slits of the phototube housing l2 so as to allow only ultra-violet light from the arc to reach the phototubes I3 and II. Then only one bafiie between arc and phototube housing would be necessary.

Each of the phototubes I3 and I4 is connected to a direct current amplifier. Two amplifiers are required, one for each phototube, and are connected as shown in Figure 2 which is the circuit diagram and includes amplifier tubes l8, I9, 20 and 2|. This circuit has proven to be satisfactory.

To the voltage divider, 450 volts D. C. are supplied from a conventional full wave rectifier supplied from 110 volts A. C. using a 5Y3G rectifier tube and a condenser input filter system. Matched 6J7G tubes are used. The leads from the phototubes to the grids of the 6J7G tubes are shielded at 22 and 23 and the shields rounded at 24 and 25. GL-868-PJ23 phototubes may be used but other types are also suitable. In particular (EL-441 phototubes are to be rec- 4 ommended, in which case a 250 volt supply may be used instead of the volt supply for the #868 phototubes. Where a filter allowing only ultra violet light is used in front of the slits of the phototube housing [2 then phototubes particularly sensitive to ultra-violet light are used.

In Figure 2 there are shown resistors Rl to R10, condensers Cl and C2 and D. C. milliammeters MI and M2. Cl and C2 are of 0.0001 mfd. capacity and RI and R2 are potentiometers each of l megohm. R3 and RI ll are resistors and have the following values:

R3, R4 4.7 megohms R5, R6 250,000 ohms (variable) R1 2,000 ohms R8 100,000 ohms R9,Rl0 25,000 ohms Ml, M2 D. C. milliammeters These values have been used by applicant but all of them, including Cl and C2 and the coils 26 and 21 mentioned below may be altered. The values for R3 and R4 will depend on the kind of arc used and the kind of sample analysed.

There are two coils 26 and 21, shown in Figures 1 and 2, each having a resistance of 4000 ohms. Laminated iron cores are shown at 28 and 29. The coils were wound in sections and balanced to give the same resistance and distribution of magnetic fiux. When equal currents flow in the two coils, the resultant magnetic field about the electromagnet is zero or nearly so. The electromagnet may be mounted with its magnetic axis coinciding with the optic axis of the spectrograph, and with the end nearest the are 6 cm. f om the are but this distance may be changed. The end nearest the arc may be insulated with asbestos board. It is convenient to construct a metal support so that the electromagnet can be swung away from the arc stand to facilitate loading and positioning of the electrodes.

It will now be clear that if the arc is properly centred, equal amounts of light will be falling on each phototube, and the currents through coils 26 and 21 will be equal, thereby producing no magnetic field. But if the arc wanders sideways then one phototube will be illuminated more than the other. This will produce unequal photocurrents, and hence unequal currents in coils 26 and 21. The magnetic field thereby produced will, be acting on the magnetic field of the are, immediately re-centre the arc. In actual practice the magnetically controlled section of the are never appears to move ofi centre. so

effective is the magnetic stabilizer.

In the phototube housing I: the preferred distance between the slit openings is such that the two slits are just inside the vertical edges of the arcs visible image, Difierent lengths of optical path from arc to phototube housing and different electrode shapes and sizes, will determine the optimum distance between slit openings. The composition of the sample can be expected partly to determine the optimum distance between slits, because the sensitivity of a phototube is a function of the wave-length of incident light. But applicant has analysed a wide variety of ores without having to change the distance. It is therefore necessary for each spectroscopist to find the most suitable distance for his particular set of conditions.

The optimum width of each slit opening will depend on the amount and wave-lengths of light reaching the phototubes. Obviously each slit UKHWIHYLR should be the same width. For a wide variety of ores the applicant has found that a slit opening 0.15 cm. wide is satisfactory. It has been found that if the slit openings are too wide, or if the slits are too far apart, or if the magnet is too near the arc, or if the amplifier has too high a gain," then an oscillation is set up in the circuit with the result that the arc oscillates sideways and appears visually as a wide fanshaped discharge, the image of which appears to cover both slits though even under such conditions wandering of the arc is reduced. Therefore to use a normally shaped are, it is necessary that each spectroscopist adjust the slits so as just to prevent the fan-shaped are forming. If a fan-shaped arc does form regardless of how narrow the slit openings are or the distance between them then a remedy would be to move the electromagnet further away from the arc. Alternatively one could replace the 4.7 megohm resistors R3 and R4 in the phototube circuit by ones with a lower resistance, or else reduce the values for R5 and R5. If the magnetic field is not strong enough to re-centre the are when it is displaced sideways, a remedy is to increase the value of resistors R3 and R4.

After closing the switches in the A. C. line the amplifier is allowed to warm up" for 5 minutes. Then resistors RI and R2 are adjusted so that equal currents fiow in coils 26 and 21 when the arc is not operating. The preferred current is one of 30 milliamperes. This value drops to 23 milliamperes when the arc is operating, and this has been found to be suitable during arcing or ore samples. From time to time, the milliammeter reading should be observed, and if unequal, should be equalized by changing RI or R2; but this adjustment should not be made while the arc is operating.

In Figure 1 may be seen the manner in which the mirror and lenses are used to produce at the photocell housing a real image a'b' of the arc ab. It should be understood that the line ab depends on the type of sample and the shape of the electrodes. It is therefore not material to the invention. The are may be about 1 cm. wide. The width ab' of the image will of course depend on the focal lengths of the lenses and these values are not material to the invention. The image normally obtained is about 5 cm. in width.

The are never is seen to move off centre; that is, the image of the are on the front wall of the photocell housing never appears to wander sideways. The arc would wander sideways without the stabilizer operating, and with the stabilizer operating it is natural to expect an extremely slight sideways shift, much too small to be observed and possibly of the order of 0.05 centimetre; but in a very minute fraction of a second, the stabilizer creates a magnetic field which shifts the arc sideways in the opposite direction of this 0.05 centimetre thus recentering the arc. Hence, it is substantially correct to say that most of the time the two phototubes are equally illuminated, and that occasionally a slight sideways shift .of the arc illuminates one phototube slightly more strongly than the other.

The magnetic strength of the electromagnet is not of importance. Normally both phototubes are equally illuminated and hence equal currents flow in opposite directions in the two coils of the electromagnet, and the resultant magnetic flux density in zero. If, as suggested above, the are moved sideways by 0.05 cm., then one photocell for a minute fraction of a second would become,

very slightly more strongly illuminated than the other, thus causing the magnet to produce a momentary magnetic field at the arc. This field immediately re-centres the arc, and then becomes zero. Hence the magnetic flux density at the arc is the important factor. It can be adjusted to a desired value by (a) adjusting the distance between magnet and are or (b) changing the values of resistors R3 and R4, or R5 and R8, and in other similar ways. Therefore the stabilizer will accommodate electromagnets of various magnetic strengths.

At all times while the direct current are is burning it will be surrounded by circular magnetic lines of force, as shown in Figure 4 where 3 is a plan of the arc, and the circles a depict the magnetic lines of force with their direction. The polar axis of the magnet is designated by the vertical line 30. With the arc centred, the resultant magnetic field of the magnet is zero that is the two coils have equal currents flowing in opposite directions as shown in the drawings.

With the are displaced say 0.05 cm. to the right one phototube is illuminated slightly more than the other. Hence the two coils of the magnet have unequal currents, and the magnets magnetic lines of force are approximately as shown in Figure 5. Hence the two superimposed magnetic fields on the left of centre partly cancel, while on the right they reinforce, thus moving the arc to the left, to centre.

When the arc is displaced 0.05 cm. to the left, the other phototube becomes more highly illuminated, and again the two coils have unequal currents; but this time the coil which previously had the lesser current now has the greater, and since the two coils are oppositely wound, the polarity of the magnet is reversed, and the resultant direction of the magnetic force is from left to right, thus centering the arc.

It is important to note therefore that, in accordance with established principles. the magnet produces stabilizing forces at right angles to its magnetic axis.

It should be understood that it is possible.us the same stabilizer equipment and arrangement heretofore described, to produce an oscillation of fairly high frequency in the amplifier circuit. This oscillation produces an oscillation of the arc itself, that is, the arc oscillates from side to side at right angles to the magnetic axis of the electromagnet. The amplitude of this oscillation of the arc can be controlled by (a) (b) (c) below, or in other similar ways. The result is that light from probably the entire width of the arc is sampled many times a second and enters the spectrograph. This sampling of the light from probably the entire width of the arc may with certain kinds of material increase the precision of quantitative spectra-chemical analysis. This oscillating arc illuminates first one photocell much more strongly than the other, then the reverse, many times a second, and in this respect is different from the heretofore described results, where both phototubes are always equally illuminated. The oscillation can be obtained by increasing sufficiently the electromagnets magnetic flux density at the arc, such as by (a) increasing the amount of light each phototube receives; (b) increasing the gain" of the amplifiers, (c) decreasing the distance from magnet to are.

While only one stabilizer unit has been described, it should be noted that by using two or more stabilizers, other portions of the arc could be stabilized. Thus, using two stabilizers. with 'one pair of phototubes immediately above the other pair, and with two electromagnets, one above the other but with their magnetic axis parallel, two distinct horizontal slices of the arc could be stabilized.

The arrangement shown in Figure 1 is used for the sake of convenience only since the apparatus may be set up in any convenient manner. It would for example be possible to eliminate the mirror 6 and the lens I and to locate the phototubes one on either side of the entrance of the spectrograph thus stabilizing the are directly rather than by reflection. Alternatively the mirror 6 and lens in can be eliminated, and the phototubes located one on either side of the grating or prism, inside the spectrograph itself.

While the invention as described so far has applied to a direct current arc, nevertheless by keeping the arrangement of parts shown in Figure 1, and by making slight alterations to the amplifiers, and by adding an extra pair of coils to the electromagnet 2 in Figure 1, then the invention can be used to stabilize an alternating cur rent arc. This is so because an alternating current arc diflers from a direct current are essentially only in that for the alternatin current are the direction of the current reversed each half cycle; hence the direction of the circular magnetic field around the alternating current are will reverse every half cycle. In Figure 3 there is shown a circuit for use with an alternating current arc. The phototubes I3 and I4 are connected to-ampliflers, these being shown at 40, 4|, 42, 43, 44 and 45; and 4| being preferably 6J7 tubes while 42, 43, 44, and may be 6L6 tubes, the leads from the phototubes to the grids of the 6J7 tubes are shielded at 46 and 41 and the shields are grounded at 48 and 49. Resistors RI l and RI9 are shown and condensers Cl and Cl2. CH and (H2 are of 0.0001 mfd. capacity while RI I and RH are 1 megohm potentiometers. R13 and RIB are resistors and have the following values.

RM, RM 4.7 megohms Rl5, RIG 250,000 ohms (variable) RI! 500 ohms (approx) R18, RIS 25,000 ohms All of these values may be altered if necessary or desired.

In Figure 3 the electromagnet comprises four coils. Coils and 5! correspond exactly to coils 26 and 21 of Figure 2. Coils 52 and 53 are another pair of coils, identical in construction with coils 50 and 5|. Hence for the alternating current are the electromagnet has four coils wound on a laminated iron core, instead of the two coils used for a direct current are.

Referring to Figure 3, it is seen that alternating current is fed to the plates of the SL6 tubes, 42, 43. 44 and 45 from the secondary of a transformer. The primary of the transformer has some common phase shifting device in it such as a condenser, shown at 54 which will bring the alternating current in the secondary of the transformer into phase with the alternating current in the arc. During one half cycle current flows through coils 50 and 5| only, and during the other half cycle it flows through coils 52 and 53. Coils 50 and 5| are wound in opposite directions so that for a centred arc the field about the electromagnet is zero; and coils 52 and 53 are likewise wound in opposite directions to each other, for the same reason. But if the arc wanders ofi centre" currents in coils 52 and 53 or 50 and BI depending on which pair is working will be unequal, and a magnetic field will result which recentres the arc and so then becomes zero. The way the electromagnets field recentres an alternating current arc is the same as has been described above for the direct current arc. Figure 4 applies to both arcs.

It may be seen by comparing Figures 2 and 3 that the operation of both amplifiers is much the same; for the direct current are, direct current is supplied to the plates of the SL6 tubes; for the alternating current arc, alternating current is supplied to those plates. Figure 1 applies to both alternating current and direct current arcs, and the operation of the invention for an alternating current are rests upon the same principles as have been described above for its successful operation upon a direct current arc.

The foregoing description is presented by way of example only, it being understood that the invention may be modified to any degree within the scope of the appended claims.

What is claimed is:

1. Magnetic arc-stabilizin apparatus for use in conjunction with a spectroscope or spectrograph and comprising; an arc, light-sensitive means and arc-stabilizing means electrically connected therewith and controlled thereby.

2. Magnetic arc-stabilizing apparatus for use in conjunction with a spectroscope or spectrograph and comprising in combination; an arc, light-reflecting means, light-sensitive means and arc-stabilizing means electrically connected therewith and controlled thereby.

3. Magnetic arc-stabilizing apparatus for use in conjunction with a spectroscope or spectrograph and comprising in combination; an are designed to transmit into a spectroscope or spectrograph light to be analysed, light-reflecting means designed to change the direction of a representative section of the beam from said are, light-sensitive means situated within the path of said reflected beam from said arc, arc-stabilizing means situated in stabilizing relationship with said arc, said arc-stabilizing means being electrically connected with said light-sensitive means and controlled thereby.

4. Apparatus as claimed in claim 2, said lightreflecting means reflecting a portion only of the beam from said arc and comprising a mirror situated angularly with respect to the direction of said beam.

5. Apparatus as claimed in claim 2, said lightsensitive means comprising at least one pair of phototubes situated within the path of the light reflected by said light-reflecting means.

6. Apparatus as claimed in claim 2, said arestabilizing means comprising at least one electromagnet.

7. Apparatus as claimed in claim 5, said phototubes being enclosed within a housing, said housing being equipped with light admitting means.

8. Apparatus as claimed in claim 2 comprising in addition a direct-coupled amplifier, the said light-sensitive means being connected through said amplifier to the said arc-stabilizing means.

9. Magnetic arc-stabilizing apparatus for use in conjunction with a spectroscope or spectrograph and comprising in combination: an are designed to transmit into a spectroscope or spectrograph light to be analysed, light-reflecting means designed to change the direction of a representative section of the beam from said arc, light-sensitive means situated within the path of 9 said reflected beam from said arc, arc-stabilizing means situated in stabilizing relationship with said arc, said arc-stabilizing means being electrically connected with said light-sensitive means v 10 11. Apparatus as claimed in claim 2, wherein means are provided for causing stabilized oscillation of the arc.

GENILLE CAVE-BROWNE-CAVE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,009,555 Mathiesen July 30, 1935 2,046,117 Guest June 30, 1936

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