US2085100A

US2085100A - Electric discharge apparatus

Info

Publication number: US2085100A
Authority: US
Publication date: 1937-06-29
Anticipated expiration: 1954-06-29

Description

June 29, 1937.

D. D. KNOWLES ET AL ELECTRIC DISCHARGE APPARATUS Filed May 17, 1935 Mas/er Shee ts-Shee t l WITNESSES INVENTORS and ATTOR Y June 29,1937. 0. o. KNOWLES ET AL. 2,035,100

ELECTRIC DISCHARGE APPARATUS Filed May 17, 1955 2 Sheets-Sheet 2 WITNESSES r Den 6 5D Know/es 5W and gY e6////0//e/T ATTORN Y INVENTORS Patented June 29, 1937 UNITED STATES PATENT OFFICE 2,085,100 ELECTRIC DISCHARGE APPARATUS Application May 17, 1935, Serial No. 22,012

11 Claims.

Our invention relates to electric discharge apparatus and it has particular relation to discharge apparatus of the type utilized for stroboscopic purposes.

Stroboscopes are commonly utilized to examine bodies in periodic motion although the stroboscopic phenomenon may be applied to bodies in aperiodic motion. One of the essential elements of a device of this type is a source of illumination the intensity of which may be made to fluctuate with a predetermined periodicity. The body under observation is illuminated by the source, preferably in the absence of other illumination, and the periodicity of the fluctuations is adjusted to correspond to the periodicityof the motion of the body. The body is thus illuminated in such manner that certain portions of it are seen only in a predetermined position and therefore appear to be at rest in this position, To accoms plish this object, the frequency of the fluctuations of the source is adjusted to equality with the frequency of the bodyor to a frequency which is equal to the frequency of the body divided by a whole number.

Since the only illumination available for observing the body in motion is preferably the lilumination of the source, it is desirable that the source emit intense radiations. It is, moreover, often desirable to take photographs of the body as it appears in the light of i the stroboscopic source. For this reason, the illuminations emitted by the source should preferably be of a type to which photographic film is highly sensitive.

In the past, electric discharge devices have been utilized as stroboscopic sources of illumination. In the envelope of such discharge devices, an inert gaseous atmosphere is provided. When the discharge device is excited, the gas emits its characteristic glow and the body under observatlon-may be illuminated by the glow.

When a discharge device of the mercury-D001 type is energized, a luminous spot forms on the surface of the mercury pool. The radiations emitted by this spot are intense and since the radiations are also rich in ultra violet light, the spot constitutes a highly desirable source for stroboscopic purposes.

The inert gas discharge devices which are utilized for stroboscopic purposes are provided with control electrodes. The potential impressed between the control electrode and the principal electrodes of a discharge device of this type is varied in such manner that the luminous glow emitted by the discharge device appears and dis- (c1. {is-'14) appears with a periodicity which corresponds to the periodicity of the motion of the body.

Mercury pool discharge devices with control electrodes or grids are available. However, such discharge devices, in general, incorporate keepalive electrodes which maintain a luminous spot continuously on the surface of the mercury elec-- trode and in view of the radiations emitted by this spot, the varying radiations produced by varying the control potential are obscured.

It is, accordingly, an object of our invention to provide for stroboscopic apparatus, a source consisting of the luminous spot produced on the surface of a mercury pool discharge device when it is energized.

Another object of our invention is to provide a stroboscopic source consisting principally of the luminous spot on a mercury pool discharge device the illumination from which shall not be obscured by radiations emitted by other elements appurtenant to the discharge device. 20

An incidental object of our invention is to provide for a. stroboscopic system incorporating a discharge device which is to be periodically energized and deenergized, a simple and inexpensive system for producing the periodic excitation of 25 the discharge device.

Another incidental object of our invention is to provide a stroboscoplc system incorporating a discharge device in which the discharge device 3 shall be periodically energized and deenergized with high precision.

More concisely stated, it is an object of our invention to provide for stroboscopic apparatus a high intensity source which is rich in actlnic 35 radiations.

According to our invention, we provide a luminous source comprising a discharge device of the type incorporating a plurality of principal electrodes and a plurality of starting electrodes 0 between which a current is passed to initiate the discharge between the principal electrodes. One of the principal electrodes is a mercury pool and the luminous spot produced on the surface of the pool, when the discharge device is energized, is preferably utilized as a stroboscopic source.

In the following discussion and in the claims, we shall refer to a discharge device having a plurality of principal electrodes and a plurality or pair of starting electrodes. Such an expression 50 p is drawn with the thought in mind that it shall denominate a discharge device having at least four electrodes, i. e., separate pairs of principal and starting electrodes.

In the preferred structure of the discharge de- 55 vice utilized in the practice of our invention, however, only three electrodes are utilized; an anode, a mercury p001 cathode and a starting-electrode, preferably composed of boron carbide or silicon carbide, a portion of which is immersed in the mercury. The mercury pool thus serves the double function of cathode and starting electrode cooperative with a boron or silicon carbide elec trode. The use of the mercury in the double capacity is, of course, in the. interest of economy and convenience and should in no way limit the scope of our invention.

While in the preferred practice of our invention the above-described discharge device is utilized, it is conceivable that discharge devices of other types may be utilized. For example, the solid starting electrode may be displaced a short distance from the pool and in. initiating the discharge a spark may be ignited between the starting electrode and the pool. Moreover, the starting electrode may be a band surrounding the envelope of the discharge device in the region of the pool. It is to be noted, however, that while discharges of the latter types may be utilized, they ofler difilculties when used for stroboscopic purposes. To ignite such discharge devices considerable and persistent starting potential is required and as far as we are aware, the operation of such discharge devices is uncertain. However, it should be kept in mind that while the discharge devices of the type incorporating the silicon or boron carbide starting electrode are preferable for stroboscopic work, where the discharge devices of the last-mentioned types have utility and where the teaching of our invention is otherwise followed such use of the discharge devices are equivalents which fall within the scope of our invention.

The novel features that we consider characteristic of our invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of speciflc embodiments when read in connection with the accompanying drawings, in which Figure 1 is a diagrammatic view showing an embodiment of our invention,

Fig. 2 is a diagrammatic view showing a modiflcation of our invention,

Fig. 3 is a graph illustrating the operation of cathode 5, and a starting electrode 1 composed of 9. high resistive material such as boron carbide or silicon carbide, a portion of which is immersed in the mercury 5. When the discharge device I is energized, as shall be explained hereinafter, the radiations emitted by the luminous spot formed on the mercury pool surface are projected on the body 9 under observation which is illustrated as the spoke of a rotating wheel II. The discharge device I is energized and deenergized at a periodicity corresponding to the periodicity of the wheel I I and when the spoke 9 is observed in the illumination emitted by the spot, it appears to be at rest.

A capacitor I3 is connected to the anode 3 of the discharge device I through an inductor I5 and directly to the cathode 5 of the discharge device. The capacitor I3 is charged through a resistor I I from a source I9 which is preferably of the direct current type. The starting electrode 1 is connected to the capacitor I3 through an auxil iary discharge device 2i and the inductor I5. The auxiliary discharge device 2i is preferably of the type having an abrupt characteristic. It is provided with an anode 23, a hot cathode 25 and a control electrode 21 which are immersed in a gaseous mediLun. The anode 23 is connected to the inductor I5 while the cathode 25 is connected to the starting electrode "I through a current limiting resistor 29. The control electrode 21 is connected to the cathode 25 through a'variable biasing battery 3I.

In operation, the auxiliary discharge device 2i is normally deenergized. The capacitor I3 is charged through the resistor I! in series therewith. When it attains a difference of potential predetermined by the magnitude of the bias in the control circuit of the auxiliary discharge device the latter is energized and a pulse of current is capacitor is recharged and the above described process is repeated.

The excitation of the main discharge device I is periodic. The periodicity of the excitation may be varied either by varying the resistor IT in series with the capacitor I3, and thus varying the rate of charging of the capacitor, or by varying the biasing battery in the control circult of the auxiliary discharge device, and thus varying the magnitude of the charge which is to be applied to the capacitor to produce excitation of the auxiliary discharge device. In the practice of our invention, either of these elements is so varied that the frequency of discharge of the main discharge device corresponds to the frequency of movement of the body 9 under observation. This correspondence is attained by subjecting the body 9 to the illumination from the discharge device and by slowly varying the resistor 01' the control battery until the body appears to be at rest.

In the apparatus shown in Fig. 2, the proper periodicity of illumination is attained by utilizing the output of a master oscillator 33, the frequency of which may be varied. Here again, a capacitor I3 is connected between the mercury pool 5 and the anode 3 of the main discharge device I. The capacitor I3 is charged from a source 34 through an auxiliary discharge device 35 of the type having an anode 31, a cathode 39 and a control electrode 4I. If the source 34 utilized for charging is of the alternating current type, the auxiliary discharge device 35 should preferably be of the asymmetric type and should be so connected to the capacitor I3 that the capacitor plate 43 which is connected to the anode 3 of the main discharge device positive.

The excitation of the auxiliary discharge device 35 is controlled from the master oscillator 33. A potential provided by the master oscillator I is charged Cir 39 is impressed between the control electrode 4| and the cathode 39 through a secondary section 45 of a suitable transformer 41 and the discharge device 35 is energized at a frequency corresponding to that of the master oscillator.

The master oscillator 33 also provides current impulses for energizing the main discharge device. For this purpose, another secondary section 49 of the transformer 41 is connected between the starting electrode 7 and the mercury pool 5 of the main discharge device I through a suitable rectifier 5i. Half wave impulses produced by the oscillator 33 are thus transmitted through the starting electrodes 1 of the main discharge device l and periodically initiate the discharge in the main discharge device, the anode cathode potential being supplied by the capacitor l3.

To provide for precise operation of the main discharge device I, the

secondaries

45 and 49 of the oscillator transformer 41 are so wound that the initiating discharge is only transmitted through the starting electrodes of the main discharge device after the capacitor l3 has been charged and when the auxiliary discharge device 35 blocks the charging current. Preferably, the potential supplied to the control circuit of the auxiliary discharge device 35 is in opposite phase to the potential supplied between the starting electrodes 1 and 5 of the main discharge device I, to accomplish this purpose.

The operation of the apparatus is illustrated in Fig. 3. In this view, the full line sine curve 53 represents the control potential supplied by the oscillator 33 to the auxiliary discharge device 35. The broken line sine curve 55 represents the control potential supplied between the starting electrodes 1 and 5. It will be noted that the full line and the broken line curves 53 and 55 are in opposite phase. The potential difference impressed on the capacitor i3 is represented by the full line curve 51 rising above the sine curves. If the resistance in series with the capacitor i3 is comparatively small, the capacitor may be charged during the interval corresponding to a single half wave of the potential supplied by the oscillator 33 as illustrated. During the succeed ing half cycle, suificient current is transmitted between the starting electrode 1 and the mercury pool 5 of the main discharge device to energize the main discharge device I and the main discharge device having been energized, the capacitor i3 is discharged.

A restifier 59 is connected between the anode 3 and the catrode 5 of the main discharge device I to prevent the current in the main discharge device from oscillating. The cathode 6| of the rectifier 59 is connected to the anode 3 of the main discharge device I and the anode 63 of the rectifier is connected to the mercury pool 5. If there is a tendency to oscillate, the rectifier network observes the oscillations.

In the apparatus shown in Fig. 4, the capacitor 13 connected between the anode and the cathode of the main discharge device is charged during a predetermined interval of time and after it has been so charged, it is discharged during a later interval of time.

For this purpose, an auxiliarydischarge device 35 similar to the discharge device utilized in the modification shown in Fig. 2 is provided in series with the power source 34 and the capacitor l3.

The cathode 39 of the auxiliary discharge device 35 is connected to the plate 43 of the capacitor l3 that is to be charged positive and the anode 31 is connected to the source. Normally, the discharge device 35 is in non-conductive condition by reason of the functioning of a biasing battery 65 connected between the cathode 39 and the control electrode H which maintains the control electrode 4i at a negative potential relative to the cathode 39. The control electrode 4| is moreover connected to a brush 6! which contacts a rotating commutator 89 provided with a conducting segment "H of predetermined length. The segment H is directly connected to the oathode 39 through a second brush 13.

As the commutator is rotated, the first brush 6'! comes into contact with the segment H for a predetermined interval of time and during this interval of time, the control electrode M is at the same potential as the cathode 39. The discharge device, therefore, passes current to charge the capacitor l3. The total resistance in series with the capacitor is comparatively low and the capacitor is charged in a short interval of time.

To ignite the main discharge device I, asecond commutator 15 mounted on the shaft 11 of the commutator 69 and rotating in synchronism is provided. The second commutator 15 is provided with a conducting segment 19 which is directly connected to the starting electrode I. A brush 8| in contact with the surface of the commutator is connected to that plate 43 of the capacitor l3 which is charged positive through a suitable impedance 83. The impedance properly regulates the starting impulse of current when it is transmitted.

The conduct-ing segment 19 on the last mentloned commutator 15 is displaced in phase with respect to the conducting segment ll of the first mentioned capacitor 69. It operates when contacted by its brush 8| to discharge the capacitor l3, after it has been charged, through the starting electrode 1 and the mercury pool 5. The charge on the capacitor i3 is not exhausted by the impulse transmitted through the starting electrode 1 and the residual difference of potential is of sufiicient magnitude to cause the main discharge device I to become energized. When the main discharge device is energized, the starting electrode circuit is shunted out. It will be seen from the above description that a small portion of the charge on the capacitor is utilized to ignite the discharge device after which the resid-' ual charge is utilized in producing the main discharge. A single capacitor serves the double function of providing starting current and discharge potential. The periodicity of the discharge produced in the main discharge device may be varied by varying the speed of the commutators*.69 and 15.

In the apparatus shown in Fig. 5, the capacitor I3 connected between the anode 3 and the cathode 5 of the main discharge device I is charged through a half wave rectifier from an alternating source 34. To discharge the capacitor I 3 through the starting electrode 7, an auxiliary discharge device 2|, similar to the discharge device utilized in the embodiment shown in Fig. 1, is provided in series with the starting electrode 1 and the cooperative mercury pool 5. The cathode 29 of the auxiliary discharge device 2| is con nected through the limiting resistor 29 to the starting electrode 1, and the anode 23 is con,- nected to the positive plate 43 of the capacitor l3.

The control circuit of the auxiliary discharge device 2| is supplied from a transformer 81, the primary 89 of which is connected across the sec-' ondary 9| of the main power supply transformer 93. However, the terminals of the primary 89 of the auxiliary transformer 81 are so arranged with respect to the terminals of the secondary 9| of the main transformer 93 that the potential supplied in the control circuit of the auxiliary device 2| is displaced in phase with respect to the potential supplied by the secondary 9| of the main transformer.

In Fig. 6, the operation of the apparatus is illustrated graphically. The full line sine curve 95 represents the potential supplied by the main transformer 93 and the broken line sine curve 91 represents the control potential supplied by the auxiliary-transformer. Since the resistance in series with the capacitor i3 and the half wave rectifier is small, the capacitor I3 is quickly charged during the half cycles during which the potential output of the main transformer 93 is such that rectifier 85 is conductive. The dot dash curve 99 represents the charging operation. After the capacitor I3 is charged and the charging operation is discontinued, the control potential supplied to the auxiliary discharge device 2| attains a value of sufficient magnitude to energize the auxiliary discharge device. The ca pacitor I3 is then discharged through the discharge device 2| and through the starting electrode l of the main discharge device l. The main discharge device I is immediately energized under the influence of the residual potential difference impressed on the capacitor and it remains energized until the capacitor is discharged. After this, the capacitor is recharged and the process is repeated. It is to be noted that here again a single capacitor I3 is utilized for the double purpose of supplying starting current and ignition potential.

Our invention has been herein shown and described as utilizing a main discharge device I in which the cathode is a mercury pool. The principal advantage of such a discharge device resides in the intense cathode spot which forms on the surface of the cathode when the discharge device is energized. Discharge devices having cathodes of other types have been utilized by us and on the surface of the cathode in certain of these discharge devices a cathode spot also forms. We have found this is particularly true of discharge devices in which the cathode is composed of such metals as cadmium, zinc, sodium, potassium, or alloys of these metals. In the claims which follow, we shall refer to a discharge device having a mercury cathode or a mercury pool cathode. We intend by this expression to cover the general case of a cathode on which a cathode spot forms and the expression should be taken to include cathodes of all types on which a. cathode spot forms.

Although we have shown and described certain specific embodiments of our invention, we are fully aware that many modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. Stroboscopic apparatus for observing a body in periodic motion comprising a discharge device having a plurality of electrodes at least one of which is mercury, means for periodically energizing said discharge device to periodically produce a luminous spot on the surface of said mercury electrode, means for projecting the radiations emanating only from said spot on a portion of the surface of said body and means for adjusting the periodicity of said energizing means to correspond to the periodicity of the motion of said body.

2. Stroboscopic apparatus for observing a body in motion comprising a discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in an unexcited condition, means for energizing said discharge device to produce a luminous spot on said mercury, said energizing means including a pair of starting electrodes adjacent to each other and means for passing a pulse of current between said starting electrodes, means for projecting the illumination emanating only from said spot on said body and means for so adjusting the excitation of said discharge device that said luminous spot is ignited and extinguished in an interval of time corresponding to the motion of said body.

3. Stroboscopic apparatus for observing a body in motion comprising a discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in an unexcited condition, means for energizing said discharge device to produce a luminous spot on said mercury, said energizing means including a pair of starting electrodes in contact with each other and means for passing a pulse of current between said starting electrodes, means for projecting the illumination emanating only from said spot on said body and means for so adjusting the excitation of said discharge device that said spot is ignited and extinguished in an interval of time corresponding to the motion of said body.

4. Stroboscopic apparatus for observing a body in motion comprising a discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in an unexcited condition, means for energizing said discharge device, to produce a luminous spot on said mercury, said energizing means including a pair of starting electrodes one of which is composed of mercury and another of a material having a resistivity which is high compared to that of mercury adjacent to each other, means for projecting the illumination emanating only from said spot on said body and means for passing a pulse of current between said starting electrodes, and means for so adjusting the excitation of said discharge device that said spot is ignited and extinguished in an interval of time corresponding to the motion of. said body.

5. Stroboscopic apparatus for observing a body in motion comprising a discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in an unexcited condition, means for energizing said discharge device to produce a luminous spot on said mercury, said energizing means including a pair of starting electrodes one of which is composed of mercury and another of a material of the type of silicon carbide having a resistivity which is high compared to that of mercury, means for projecting the illumination emanating only from said spot on said body and means for so adjusting the excitation of said discharge device that said spot is ignited and extinguished in an interval of time corresponding to the motion of said body.

6. Stroboscopic apparatus for observing a body in periodic motion comprising a discharge device having a plurality of principal electrodes at least one of which is mercury, said mercury electrode being normally maintained in an unexcited condition, means for periodically energizing said discharge device to periodically produce a luminous spot in the surface of said mercury electrode in the illumination from which said body is to be observed; said energizing means including a pair of starting electrodes, one of which is composed of mercury and the other of a material, such as boron carbide, having a resistivity high compared to mercury, in contact with each other and means for periodically passing a pulse of current between said starting electrodes, means for projecting the illumination emanating only from said spot on said body and means for adjusting the periodicity of said energizing means to correspond to the periodicity of the motion of said body.

'7. Stroboscopic apparatus for observing a body in periodic motion comprising an electric discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in uncxcited condition and a. plurality of starting electrodes one of the latter being composed of mercury and the other of a material such as boron or silicon carbide, a portion of which is immersed in said mercury, means for supplying a potential between said principal electrodes, a capacitor, means for charging said capacitor at a predetermined rate, a second electric discharge device of the type having an abrupt characteristic, means for connecting said capacitor, said second discharge device and said starting electrodes in series, whereby when said capacitor is charged to a predetermined potential, said second discharge device is energized, a current is transmitted between said starting electrodes, a discharge be tweeen said principal electrodes is initiated and a luminous spot is produced on the surface of the mercury principal electrode, said discharge being extinguished after a. predetermined interval of time, means for projecting the illumination emanating only from said spot on said body and means for adjusting the rate of charge of said capacitor to such a magnitude that the periodicity of ignition and extinction of said discharge correspond to the periodicity of the motion of said body.

8. Stroboscopic apparatus for observing a body in periodic motion comprising an electric discharge device having a plurality of principal electrodes at least one of which is composed of mercury said mercury electrode being normally maintained in unexcited condition and a plurality of starting electrodes one of the latter being composed of mercury and the other of a material such as boron or silicon carbide, a portion of which is immersed in said mercury, means for supplying a potential between said principal electrodes, a capacitor, means for charging said capacitor at a predetermined rate, a second electric discharge device of the type having an abrupt characteristic, means for connecting said capacitor, said second discharge device and said starting electrodes in series, whereby when said capacitor is charged to a predetermined potential, said second discharge device is energized, a current is transmitted between said starting electrodes, a discharge between said principal electrodes is initiated and a luminous spot is produced on the surface of the mercury principal electrode, said discharge being extinguished after a predetermined interval of time, means for projecting the illumination emanating only from said spot on said body and means for so adjusting said second discharge device that it becomes energized when a predetermined potential difference has been impressed on said capacitor, the magnitude of said potential difference being such that the time required for charging the capacitor corresponds to the periodicity of the motion of said body.

9. Stroboscopic apparatus for observing a body in periodic motion comprising an electric discharge device having a plurality of principal electrodes at least one of which is composed of mercury, said mercury electrode being normally maintained in deenergized condition and a plurality of starting electrodes one of the latter being composed of mercury and the other of a material such I as boron or silicon carbide, a portion of which is immersed in said mercury, means for supplying a potential between said principal electrodes, a capacitor, means including a resistor for charging said capacitor at a predetermined rate, a second electric discharge device of the type having an abrupt characteristic, means for connecting said capacitor, said second discharge device and said starting electrodes in series, whereby when said capacitor is charged to a predetermined potential, said second discharge device is energized, a current is transmitted between said starting elec trodes, a discharge between said principal electrodes is initiated and a luminous spot is produced on the surface of the mercury principal electrode in the illumination from which said body is to be observed, said discharge being extinguished after a predetermined interval of time, means for projecting the illumination emanating only from said spot on said body and means for varying the magnitude of said resistor to adjust the rate of charge of said capacitor to such a magnitude that the periodicity of ignition and extinction of said discharge correspond to the periodicity of the motion of said body.

10. Stroboscopic apparatus for observing a body in periodic motion comprising an electric discharge device having a plurality of principal electrodes at least one of which is composed of mercury said mercury electrode being normally maintained in unexcited condition and a plurality of starting electrodes one of the latter being composed of mercury and the other of a material such as boron or silicon carbide, a portion of which is immersed in said mercury, means for supplying a potential between said principal electrodes, a. capacitor, means for charging said capacitor at a. predetermined rate, a second electric discharge device of the type having an abrupt characteristic, said second discharge device having a control electrode and a plurality of principal electrodes, means for impressing a potential difference bemeans for connecting said capacitor, said second discharge device and said starting electrodes in series, whereby when said capacitor is charged to a predetermined potential, said second discharge device is energized, a current is transmitted between said starting electrodes, a discharge between said principal electrodes is initiated and a luminous spot is produced on the surface of the mercury principal electrode, said discharge being extinguished after a predetermined interval of time, means for projecting the illumination emanating only from said spot on said body and means for adjusting the potential impressed between the control electrode and the principal electrode to a value such that said capacitor has been charged to a predetermined difference of potential, the magnitude of said potential difference being such that the time required for charging the capacitor corresponds to the periodicity of the motion of said body.

11. Stroboscopic apparatus for observing a body in periodic motion comprising an electric dis- 5 charge device having a plurality of principal electrodes at least one of which is composed of mercury said mercury electrode being normally maintained in an unexcited condition and a plurality of starting electrodes one of the latter being com- 10 posed of mercury and the other of a material such as boron or silicon carbide, a portion of which is immersed in said mercury, a source of electrical oscillations, means for supplying a potential be tween said principal electrodes, said means com- 15 prising a capacitor so connected that the difference of potential impressed on it is substantially impressed between the principal electrodes of said discharge device and means for charging said capacitor including another electric discharge de- 20 vice having a control electrode and a plurality of principal electrodes and means for coupling said source of electrical oscillations between the control electrode and a principal electrode oi said last-mentioned discharge device to control the excitation thereof, additional means for coupling said source of electrical oscillations between said starting electrode, whereby periodic impulses of current are transmitted through said starting electrodes and a discharge is initiated between the principal electrodes of said first-mentioned discharge device and a luminous spot is produced on the surface of said mercury principal electrode, the output of said first-mentioned coupling means being so related in phase to the output of said additional coupling means, that in any one cycle, charging current is transmitted to said capacitor while the discharge in said first-mentioned discharge device is initiated and is interrupted when the discharge in said first-mentioned discharge device is about to be initiated, means for project ing the illumination emanating only from said spot on said body and means for adjusting the frequency of said source of oscillations to a value corresponding to the frequency of said motion. DEWEY D. KNOWLES. CECIL E. HALLER.