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A thermionic trigger

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1938 J. Sci. Instrum. 15 24(http://iopscience.iop.org/0950-7671/15/1/305)
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-4. HICKLING AKD J, H. BRUCE
24equal to the peak voltage of the supply. If, however, it is shunted by a resistance, this will
lower the maximum potential attained, and also introduce a ripple of somewhat complicatedwave-form owing to the partial discharge of the condenser while the rectifier is not conduct.
was connected through a small Westinging, In the present experiment, a 16 p ~condenser
house rectifier t o a transformer giving about 20 V. peak, and was shunted by various resistances. The maximum and minimum voltages across the condenser were measured withthe thyratron potentiometer, and are shown in Fig. 2, plotted against the reciprocal of theshunting resistance. From this the effect of load, both in diminishing the maximum potential and increasing the ripple, can be seen. These results were substantially confirmed byexamination with the cathode-ray oscillograph; this method is, however, less suitable than
the thyratron potentiometer for the accurate quantitative investigation of such problems.
The authors’ thanks are due to Dr L. G. EI. Huxley for encouragement and advice.

A THERMIONIC TRIGGERBY OTTO H. SCHMITT, PH.D., University College, London[IWS. received 26 11-ovember 19371

ABSTRACT. A simple hard valve circuit is described which provides positive off-on control
with any desired differential from 0.1v. to 20 v. Less than IO-^ amp. is required at the input,
but up to 20 ma. at zoo v. is available in the output. Either positive or negative control is possible.
The operation cycle occupies about I O p sec. Applications to cathode ray oscillography, to“ thermostating ” and to lighting control are illustrated.

THERE
are numerous electrical systems in which it is desired that a slowly varying potentialfrom some control device shall actuate another circuit in a positive, ‘ I on-off” manner. Usuallyeither a gaseous valve or a mechanical relay is employed, with or without amplification.
While these methods are quite satisfactory for many purposes, they do not give the quickness of action and flexibility of control whichmodern design demands. The thyratron operates
quickly enough but requires some special means of de-ionization, has no inherent differential
characteristic, and is notoriously subject to variation of characteristic with time and temperature. Besides, it consumes much cathode heating power and requires a “warm-up ” period
before anode potential may be applied. The relay, while it is dependable, is necessarily
sluggish and insensitive, and has a limited range of differential with any given design.
The method here suggested, which is similar to certain trip relay circuits,* is to use a hardvalve circuit into which has been incorporated an adjustable differential trigger action. The
anode current starts suddenly (within 1-10 p sec.) after an inferior critical input potential
level has been reached, and remains at full strength until a superior critical potential isreached, whereupon the system reverts to its former condition equally quickly.
As will be noted on reference to the graph (Fig. z ) , the inferior or on” critical potential issubstantially constant, while the superior or off” potential is readily and linearly adjustableby means of the rheostat R. Thus the differential between “ o n ” and “off” may be variedsmoothly from zero to more than 20 v. without affecting instantaneous positive action.
T h e circuit (Fig. I ) is substantially that of a two stage direct-coupled amplifier of simple
design with the addition of a rheostat R in the common cathode circuit. The action can be seenroughly by considering the circuit as two separate amplifiers directly coupled. A positive
increment of potential to the input grid causes a large negative increment to the second
grid’s potential and, it is true, a slight increment in its own grid bias which tends to minimize
the increase in first anode current. The second valve however experiences a very large
decrement in current and consequently causes a large decrement in first tube bias, thus

* e.g. E.T.Z. 53 1932 (669).

A THERMIONIC TRIGGER
25
rendering it unstable and tripping the trigger ”. The inverse operation occurs when,with a sufficiently negative input voltage, the positive potential returned from the second~ l v ise finally overpowered.
4-T’i,w

Fig. 2. Differential trigger characteristicFig. I . Thermionic trigger circuit

More quantitatively, an applied input voltage E$, causes a change of cathode voltage Ek ofbutwhere MIis the overall mutual conductance of the first triode, given approximately by theM2 is a similar expression for the second triode; A is the net amplification of the first valve
given approximately by
RZI
R4
A=pL1R X , R,+R,’E,, is the change of potential at the first grid, Ral and R,, are the anode resistances, Rpland R, the load resistances, and Mal and Magare the mutual conductances respectivelyof the first and second valves. pL1is the amplification factor of the first valve.
Equating the two expressions for Ek we findNow usually M, < M2and A > I , so the second term in the denominator may be neglected.
I t is then obvious that the system passes out of equilibrium when AM2R>I. Negativevalues indicate that equilibrium is not yet established, so the system will shift to new locialong the A , &I,, -44, characteristics until the system just balances, hence the differential.
I n a circuit now in use, the following set of constants is employed :p1=pLn=36, ! i , , = M a = 1 * 5 millimhos, R a 1 = R a Z = 2R.,
~ , RS1=~o’Q.,~~~

Ra=104Q., R , = j x I O S Q . ,R4=2x 1o6R. and negative bias=zoov.

0. H. SCHMITT

26(This is for the type 6A6 valve; almost any triodes or pentodes may be used.) From thisdata +MI= 0.2; millimho, M, = I millimho, and A = 24 ; hence the critical value of R, namely
R, ,is given byII

R, =-=42 Q.

AfW2-M1 2 4 x 1 0 - ~ - 2 - j X I O - ~Experimentally this value is verified, well within the limit of error, as 4j 0.
Uses for the circuit are at once obvious. I t may be used to control thermostats, refrigeration, lighting, chemical reactions ; all using the adjustable differential trigger in conjunction
with some source of potential which varies continuously with the phenomenon to becontrolled. It gives convenient accept-reject indication for mass production checking andprovides convenient photoelectric selection and counting.
In other cases its fast action is the specially attractive feature. In cathode-ray oseillography it provides convenient sweep circuit synchronization, while in the laboratory- itprovides an easy method for producing rectangular A.C. waves of any desired frequency
and phase and, in another application, acts as a frequency meter more linear than one of the
thyratron type, and one immune to locking.
Since output may be taken from either triode, and one is on while the other is off,
obviously either positive or negative control may be had, although somewhat less power
may be taken in the first case. In the normal connection, using small receiving triodes,
up to 20ma. at 2 0 0 ~may.
be taken, but much larger amounts may be had if large valves
are used or several are paralleled; in any case, the input power need not be more than
0.2 V. at less than a microampere. Voltage operated loads may be connected to either anodeor between cathode and earth ; current operated devices should be inserted in series witheither the cathode or an anode circuit.
Operation is gratifyingly stable and is quite insensitive to supply line variations provided
reasonable constants for the various circuits are chosen. Under typical conditions, bothcritical potential and differential may be depended upon to stay within IOO mv. under all
ordinary conditions.

LABORATORY AND WORKSHOP NOTEST H E ZERO SHIFT IN SENSITIVE MOVING-COIL GALVANOMETERS. BY IT.VOSS, M A , PH.D. .4KD M. N. S. IMAIIELMAK, M.Sc.,
University of Pretoria, South Africa[MS. received 1 1 October 19371

THEshift of zero in galvanometers is a bugbear in every physical laboratory. When, forinstance, the coil of a certain ballistic galvanometer in this laboratory was given a “kick” of23 cm. on the scale, to the right, then immediately short-circuited and the position of rest of
the reflected beam noted, and then this was repeated with the “kick” to the left, the tvio
positions of rest differed by nearly 2 cm. If, however, instead of short-circuiting the coilafter a “kick”, the galvanometer circuit was closed through a resistance of about 1000Q.,
the coil would swing to and fro several times and the spot of light would always come torest a t the same point. That the shift of zero on short-circuiting the coil was not due to thetemper of the suspension-strip having been affected by the heat of the soldering-iron wasshown by replacing the soldered joints by tiny screw-clip joints. The shift of zero wasthen as evident as before.
It was suggested by Prof. R. W. Varder and Prof. W. H. Logeman, with whom the