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A Valve Amplifier for Ionisation Currents

Published online by Cambridge University Press:  24 October 2008

C. E. Wynn-Williams
C. E. Wynn-Williams
Affiliation:
Trinity College
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Abstract

A method of using a valve for amplifying ionisation currents 100,000 times is described, which avoids the instabilities usually associated with such apparatus. The necessary conditions to be satisfied, while extremely simple, are based on theoretical conditions which are discussed in full. Used in conjunction with a galvanometer of sensitivity 200 mm. per micro-ampere, the system behaves in a similar manner to a low capacity quadrant electrometer of sensitivity 6350 mm. per volt, shunted by a leak of 360 megohms, the value of the latter being slightly greater for negative currents than for positive. In addition, it has the advantages of portability, compactness, and ease and rapidity of erection and operation, being specially suitable for lecture demonstrations.

The method of compensation can also be applied to other valve circuits, resulting in a much steadier zero.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 23 , Issue 7 , July 1927 , pp. 811 - 828
Copyright
Copyright © Cambridge Philosophical Society 1927

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References

* Provided that the valve is operated on a suitable portion of its triode characteristic.Google Scholar

* In this paper the filaments are assumed to be connected in parallel. By using shunt resistances, compensation can also be effected when the filaments are connected in series.Google Scholar

* While in this method, the value of is adjusted to equal the existing value of , the converse method is also possible, and sometimes more convenient. The ratio of is fixed by clamping down the slider B in one position, and the characteristic curves are searched by raising the voltage across AB until a pair of points P and Q are found for which the ratio is equal to the predetermined ratio , indicated by a reversal in the galvanometer deflection.Google Scholar

* Where great steadiness of zero is required, and a large capacity h.t. battery is available, simultaneous compensation may be effected by using a common l.t. and h.t. battery. The slider B [Fig. 1(c)] should be connected through a resistance of about 150 to 300 ohms to the positive pole of the h.t. battery. As before, compensation is effected by varying the position of the slider B on EF until a small change in the voltage of the battery produces no debalancing. In this case, the characteristic curves on which P and Q lie are represented by a diagonal section of the surfaces of Fig. 3, the l.t. and h.t. being both represented proportionally on the horizontal axis. With this system, the size of the common 60 volt battery required to supply a filament current about a hundred times the value of the plate current is rather a drawback.Google Scholar

* The similarity is earned further by the fact that the perfectly insulated grid of a hard valve would retain a negative charge indefinitely. In such a case, a negative ionisation current to the grid would result in a steady increase in the potential of the latter, giving rise to a regular movement of the galvanometer. The current could, in this way, be measured in a similar manner to that employed for an electrometer without a leak. Such a system might, however, be unstable without a leak to earth of some sort.Google Scholar

* This shows that the valve is operated on a suitable portion of its triode characteristic carve.Google Scholar

† As one of the two valves employed is sometimes found to be more sensitive than the other for current amplification, it is advisable to try both in turn and observe which is the better. The small differences in grid current characteristics and hardness, which affect the sensitivity, are not usually apparent in the tests to which valves are subjected by the makers.Google Scholar

* This includes leakage of the valve cap and valve holder, etc.Google Scholar

* An ionisation or other saturation leak would have to be used to smooth out fluctuations of the bias battery voltage.Google Scholar

† This type of apparatus has been tried by Mr J. A. Batcliffe for signal-strength measurements, and found to be very useful for this type of work, being superior to an ordinary valve detector on account of its steady zero, and less troublesome than a crystal rectifier.Google Scholar