Patent GB-491587-A (GB)
Title: Improvements in and relating to a device for tracing the trajectories of electrically charged particles in electric or magnetic fields
IPC: G06G7/48
Filed: 1937-01-05
Abstract: 491,587. Electric tests. BRITISH THOMSON - HOUSTON CO., Ltd., and GABOR, D. Jan. 5, 1937, No. 256. [Class 37] A device for tracing the trajectories of electrons or other electrically charged particles in electric or magnetic fields in vacuum devices, comprises an electrolytic bath constructed as an enlarged scale model of the device, alternating potentials proportional to those existing in the vacuum device being applied to the electrodes. A charged particle is represented by a group of probe electrodes partially immersed in the electrolyte and also by a recorder movable over a plane surface by means of a trolley mechanically connected to the probes so that the recorder and the point of the group of probes representing the particle move in similar paths, the trolley being steered according to electrical measurements derived from the probe electrodes so that the curvature of the recorded path corresponds to that required. Correction is made for the error introduced by polarization at the electrodes. Fig. 1 shows the electrostatic field and equipotential lines existing between a cathode 1 and anode 2, the thicker line showing the theoretical path of an electron emitted from the cathode. At any point P in its path, it is shown in the Specification that the radius of curvature of the path is given by r/4#=V/(V1+ V2) where r is the radius of curvature, V is the potential between the electrode 1 and point P, and V<1>, V<2> respectively, the potential drops across the small distances # between points Q1, P and PQ2 on a path perpendicular to the track. The curvature could be determined from this equation by a geometrical construction, Fig. 2 (not shown), but according to the invention the following method is used. Three probes mounted in a glass stem dip into the electrolytic bath 13, Fig. 5, at the points Q1, P, Q2 and are connected, through a pantograph pivoted at 17, to a trolley 9 mounted on three wheels 10, 11, 12, and carrying the tracing- pencil P1 over the drawing board 15. The wheels 10, 11 are freely rotatable about a fixed axis (relative to the trolley) in which lies the pencil P1 and which is always kept parallel to the line Q1, PQ2 by the linkage 18 ... 21. The trolley wheel 12 is rotatable about a vertical axis and rotates the trolley about a centre C1. C1' P1 therefore =d/tanalpha where d is the perpendicular distance of the vertical axis of wheel 12 from C1, P1 and alpha is the angle between the horizontal rotational axis of wheel 12 and C1, P1. If the pencil P1 is to record the correct path, C1, P1 must be equal to the radius of curvature n referred to above, from which tan alpha = d/4#.V1+V2/V. Determining alpha. The bridge shown in Fig. 6 is used for this purpose and comprises an A.C. source feeding a potentiometer wire AB from which sliders S<1>, S<2>, supply the potentials to the electrodes in the electrolytic bath (1, 2, Fig. 1) while S‹ supplies a potential at which the electrons have zero velocity, this potential differing slightly from that of the cathode and correcting for the initial velocity of emitted electrons. A potentiometer wire 25 connects points O, B, and a " tangent bridge " potentiometer wire 24 is mounted along the hypotenuse of a right-angled triangle O<1>O<2>M and coacts with a slider O<2>P<2> pivoted at O<2>, and connected through an A.C. zero instrument I and switch K to the probe P, the probes Q1, Q2 being connected to the bridge circuit through a transformer TR. The sliders O<2>, P<2> and G are set so that the instrument I reads zero whether the switch K is opened or closed, in which case the resistance of the wire O<1>P<2> is proportional to V, the resistance P<2>M is proportional to n2/n1(V1+ V2), where n2/n1, is the transformation ratio of the transformer, and, provided that the angle B shown is made such that tan B =n1/n2.d/4# the angle P<2>O<2>M is the angle alpha required. Transmitting #<alpha> to the trolley, Fig. 7. The trolley 9 has a disc 31 rigid therewith linked at 30, 29, 28, to a similar disc 27 rigid with the probe assembly 7. The slider O<2>P<2>, Fig. 6, forms the arm 42 and extension 44 of a differential gear comprising idler gears 36, 39 coaxial with the axis 40 of the arm 42 and wheels 37, 38 rigid with each other. The wheel 35 is rigid with a wheel 33 operated by the link 29 and the wheel 47 is rigid with a wheel 48 linked at 49, 50 to a wheel 51 rotatably mounted on the trolley and geared at 52, 53 to rotate the steering wheel 12 about its vertical axis. The arrangement is such that, with the slider 42, 44 stationary, the steering wheel 12 will always turn with the trolley 9 without changing its position relative thereto, that is #<alpha> remains constant, but when the differential-slider arm 42, 44, is adjusted to a new position the steering wheel 12 will be correspondingly adjusted. Operation. The electron paths are therefore plotted by adjusting the sliders O<2>, P<2> and G of Fig. 6 until the instrument reading is zero for both positions of switch K. The steering wheel now has the correct position and the trolley may be moved a short distance, after which the setting of the bridges, Fig. 6, is repeated and the trolley again moved, and so on. Corrections. Polarization at the electrodes in electrolytes introduces errors in bridge measurements of probe electrode potentials associated therewith. Thus Fig. 9 shows an equivalent electric circuit where an A.C. source of potential is applied across a bridge wire 23 and an electrolyte represented by resistance R in series with capacities C1, C2 representing the polarization sheaths at the electrodes. The potential drop across the wire 23 is represented by AB, Fig. 10, and AC, DB, represent the potential drops across the capacities C1, O2, at right-angles to CD the potential drop across the electrolyte which acts as a resistance. It is seen that if a, probe P in the electrolyte is connected to a slide Sp on the bridge wire, balance can only be obtained at the point wher AB and CD intersect. To remove this source of error, according to the invention, for any point along the line CD the point corresponding to the projection thereof on AB is found, and taken as the potential of that point. Thus, if the two circuits of a wattmeter are connected, one across AB, Fig. 11, and the other between the probe, for example, at D and the slider at D<11>, say, then the wattmeter will indicate the product AB x DD<11> x cos # and by adjusting the slider until the reading is zero, that is when cos# = O, the point D<1> is obtained where DD<1> is perpendicular to AB. The projections on AB of any other point in CD is similarly obtained, and, according to the invention, the potentials of the probe electrodes in the bridge shown in Fig. 6 are measured in this way by replacing the zero instrument I by a wattmeter. Wattmeters. Fig. 12 shows a valve wattmeter for use as the zero-instrument described above. One of the circuits is applied at c through a transformer TR<1> and amplifier to the grids of two valves the voltage E of the other wattmeter circuit being fed to the valves by cathode injection as shown, the D.C. instrument in anode circuit indicating E.e.cos #.