Exposure Meter for CathodeRay Oscilloscope PhotographyJ. P. Keene Citation: Review of Scientific Instruments 32, 1143 (1961); doi: 10.1063/1.1717184 View online: http://dx.doi.org/10.1063/1.1717184 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/32/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in CathodeRay Oscilloscope Intensity Modulator Rev. Sci. Instrum. 26, 1120 (1955); 10.1063/1.1715201 Precision Measurements with a CathodeRay Oscilloscope Rev. Sci. Instrum. 23, 701 (1952); 10.1063/1.1746143 CathodeRay Oscilloscope Camera Rev. Sci. Instrum. 19, 471 (1948); 10.1063/1.1741299 Continuously Calibrated CathodeRay Oscilloscope Rev. Sci. Instrum. 18, 851 (1947); 10.1063/1.1740862 A Low Capacity Coupler for the CathodeRay Oscilloscope Rev. Sci. Instrum. 11, 375 (1940); 10.1063/1.1751588

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THE REVIEW OF SCIENTIFIC INSTRUMENTS

Notes

BRIEF contributions in any field of instrumentation or technique within the scope of the Journal can be accorded earlier publication if submitted for this sution. Contributions should in general not exceed 500 words.

Zener Diodes as Coupling Elements in Relay Circuits

FERDINAND NIBLER

Institut fiir Technische Elektronik, Technische Hochschule Miinchen, Munich, Germany

(Received April 17, 1961; and in final form, August 7, 1961)

IN control circuits, e.g. thermostats, quite often the following problem arises: whenever a controlled vari­

able x, e.g., the temperature, is reaching a certain value Xo,

a relay has to operate, and when x changes to xo+Ax, the relay has to release. In the majority of cases Ax should be as small as possible for the most accurate controlling of x.

There is quite a difference, however, between the operate energization of a relay and the release energization (about 3:1·· ·2:1),1 therefore both the relay current and the variable x would have to change likewise over a wide range to make the relay "on" or "off." To avoid these difficulties in a laboratory-built thermostat (see Fig. 1), the tempera-

RI

sz. It C2

RIo It/lOmA

JOV-

~---------------------------o. I5V·

FIG. 1. Zener diode (SZ6) as coupling element in a temperature con­trol circuit. A change of 0.5 mv of base voltage of the transistor TF 65 will operate or release the relay, corresponding to a change of tem­perature of SXlO-3 °C at 35°C. R1 is adjustable to different working temperatures; C2 was chosen experimentally between 10 and 100 pf.

VOLUME 32, NUMBER 10 OCTOBER, 1961

of T2 will operate or release the relay, and, of course, an even smaller one of base voltage on T1 and therefore of x, the variable to be controlled.

Though it seems possible to use a Zener diode alone, in series with the relay, to achieve the above-mentioned threshold, it is advisable to use an additional amplifier to keep ~x small not only relatively but also absolutely. The control circuit in Fig. 1, for instance, needs only a change of base voltage of T1 of about O.S mv to operate or release the relay.

The components of the circuit are not at all critical. Only the capacitor C2, stabilizing the working point, is chosen experimentally between 10 and 100 I-'f.

1 H. Petzold, Das Fernmelderelais und seine Sclzaltung (Winter'sche Verlagsbuchhandlung, Fiissen, 1952).

2 C. Zener, Proc. Royal Soc. (London) 145,523 (1934). 3 K. G. McKay and K. B. McAfee, Phys. Rev. 91, 1079 (1953).

Exposure Meter for Cathode-Ray Oscilloscope Photography

J. P. KEENE

Research Physics Department, Christie Hospital and Holt Radium Institute, Manchester, 20, England

(Received June 26, 1961)

T HE intensity of a cathode-ray tube spot usually varies with the settings of the focus and astigmatism

controls as well as with the brightness control and it is not easy to precalibrate all three to obtain correct exposures for a wide range of spot velocities. The exposure meter de­scribed here overcomes this difficulty by integrating the light output of the trace. It consists of a small vacuum photocell mounted adjacent to the camera lens with its cathode facing the screen. Current passed by the cell charges a condenser and the resulting voltage is measured with an electrometer. In this way the total charge pro­duced by the light from a single sweep of the spot, or the rate of production of charge by a repetitive trace, can be determined. The actual charge required to produce a good photograph is a constant and need only be determined once. Then to obtain the correct brightness for a photo­graph of a single sweep it is only necessary to adjust the oscilloscope to get this charge at whatever sweep speed is in use. A check measurement can also be made when the photograph itself is taken, to confirm that conditions have not changed. To photograph the graticule or a repetitive

ture sensitive part (a Wheatstone-type bridge with a trace, a brightness which requires a few seconds exposure thermistor) and the relay were coupled by a transistor can be chosen and the camera shutter left open until the amplifier and a Zener diode driven near the Zener point.2,3 desired charge has accumulated. An artificial threshold was thus achieved resulting in two The charge produced when the screen emits the amount distinctly separated modes of operation for the relay, i.e. of light required for a photograph depends on various "on" and "off". A small change of voltage on the collector geometrical factors and also on the phosphor color, photo-

1143

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1144 NOTES

cathode sensitivity, and film speed. A Cintel VA16 photo­cell produces about 2X 10-10 coul when placed 15 cm from a P1 screen, if the brightness is adjusted so that a single sweep gives a good photograph with an 1/3.5 lens and a 1000 ASA film at unit magnification. This quantity of charge gives 2 v on a 100-J.'J.'f condenser and can easily be measured with an electrometer voltmeter. About 50 v is required to saturate the photocell, and in order to mini­mize leakage current it is advisable to remove the cell from its base, solder connections direct to the wire leads, and paint a colloidal graphite guard ring on the glass surface around the anode lead. This procedure reduces the sum of leakage and dark currents to less than 10-!4 amp. A simple two-valve battery-operated electrometer circuitl

is adequate for the charge measurements and will also supply the photocell cathode voltage. The circuit and indi­cating meter can be built into a small box mounted on top of the oscilloscope and connected to the photocell via a short piece of coaxial cable and a separate h.t. (negative) lead. A switch operated by opening the lid of the camera hood and connected so as to short the electrometer input is useful in avoiding excessive signals due to room light.

The use of a photocell in the way desuibed makes no allowance for the increased spot velocity and consequent reduction in brightness if parts of the trace have large vertical components of direction. To obtain a correct exposure for these the brightness must be increased to give a charge which is larger than the calibration value by the factor sec8, where 8 is the angle between the horizontal and the part of the trace to be correctly exposed. The factor can usually be neglected for angles less than 45°. Calibration carried out with a single sweep at 1(}'! cm/sec is satisfactory for most purposes although, in principle, reciprocity failure should then cause incorrect exposure at very high and very low speeds. We have not investigated speeds less than 1 cm/ sec and it is possible that these may require separate calibration. This might also be necessary at high speeds with short afterglow phosphors but the relatively long decay time of the P1 phosphor avoids this difficulty and exposures at a speed of 5X10· cm/sec give reasonable photographs. If films faster than 1000 ASA and lens apertures greater than about 1/3.5 are used the phosphorescence caused by exposure of the screen to room light produces a photocurrent which takes several minutes to fall to a manageable level after closing the hood lid. This effect can be reduced by operating with dim room lighting. Similar currents result if the trace is brightened excessively (or if the graticule is fully illuminated) in the course of setting up. Despite these minor difficulties the exposure meter described has been found to perform satis­factorily in practice and is a great help in obtaining properly exposed photographs.

1 J. P. Keene,]. Sci. Instr. 37, 218 (1960).

Refractory Metal Wire Cloth for High Temperature Porous Structures*

G. KUSKEVICS

General Electric Company, Cincinnati 15, Ohio

(Received June 15, 1961; and in final form, July 26, 1961)

VERY fine metal wire cloth of tantalum, molybdenum, columbium, and tungsten has been produced! for

cesium ion emitters for operation at 1200-1600oK. Meshes of 20X200 and 50X650 of Twill Dutch weave have been used, but an improvement to 325X2300 mesh is possible. A lX3 ft piece of tantalum cloth is. illustrative of the size. The cloth is optically opaque and its porosity is much more reproducible than that of a sintered material. The porosity or permeability can be controlled by compression or rolling of the cloth.2 This cloth could be used wherever a strong, flexible, high temperature porous material is required, such as in porous cathodes, porous arc electrodes, rocket chamber walls, reentry nose cone surfaces, high tempera­ture filters, etc. The weaving was done on the commercial loom by Newark Wire Cloth Company and Unique Wire Weaving Company. Other weavers interested in this field are the Cole-Roscoe Manufacturing Company, which has a sample loom, and Kassel Export Company which has produced the 325 X 2300 mesh in stainless steel.

* This work was partially supported by Marshall Space Flight Center, NASA, Huntsville, Alabama.

1 G. Kuskevics, "Extra fine refractory metal cloth for ion emitters," General Electric Rept. No. R61FPD26, January 1961 (available upon request).

2 G. Kuskevics and B. C. Merten, "Wire Cloth Conductance for Inert Gases," General Electric Rept. No. R61FPD86, January 1961 (available upon request).

Enhanced Hydrogen Pumping with Sputter-Ion Pumps

S. L. RUTHERFORD AND R. L. jEPSEK

Varian Associates, Palo Alto, California

(Received June 16, 1961; and in final form, August 1, 1961)

T HE pumping mechanism for such chemically active gases as oxygen and nitrogen in cold-cathode gas

discharge getter-ion pumpsl is dominated by chemical combination with sputtered active metal atoms from the cathodes.2- 4 These gases cause cathode materials such as titanium or zirconium to be sputtered rapidly enough that approximately one atom of gas is pumped for each ion formed in the discharge. Most of the pumped oxygen and nitrogen reside on the anode of the pump (rather than on the cathodes) in chemical combination with sputtered cathode material.

In contrast, hydrogen is so light that its ions produce comparatively little sputtering. Despite this, it is found that present sputter-ion pumps employing cathodes of

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