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The instrument is quite dead beat and very quick acting, so that it is able to follow currents varying over a small range as rapidly as one or two cycles per second.

The instrument is very robust and can be carried about in the pocket, is easily set up and requires no levelling, and has an extremely small self-induction and capacity.

It has a wide range of usefulness, and can be used (with suitable resistances) as a voltmeter measuring from 0'1 volt to 10,000 volts, or as an ammeter to measure small or large currents.

The deflections are approximately proportional to the square of the current. With a recent instrument the smallest measurable current was 6'0 X 10-3 amp. ; and the smallest detectable current 0’6 X 10-3 amp.

String Electrometer.

(1) " A String Electrometer,” by T. H. Laby, Proc. Cambridge Phil. Soc., Vol. XV., Part II.

(2) " Electrician” (London), August 20th, 1909.

This is a delicate instrument for measuring changes of potential and consists of a silvered quartz fibre stretched mid-way between two vertical plates of invar steel. Initially the fibre is at zero potential, and one plate at + 40 volts, and the other at —40 volts. A change of potential of the fibre causes a lateral deflection of it, which is measured by means of a micrometer microscope. The insulation throughout is of quartz and mica. The special features of this instrument are (1) the means employed to obtain a constant but adjustable tension on the fibre; (2) the insulation ; and (3) the microscope and mounting.

The sensitiveness depends on (1) the distance of the invar plates apart; (2) their potential difference; (3) the period of the fibre, which depends upon its tension.

By the use of invar plates, a geometrical knife-edge, and a screw adjustment, the sensitiveness of the instrument may be quickly altered within a wide range, or be kept constant at some desired amount. A sensitiveness of 600 eye-piece divisions per volt has been obtained.

A 4240.

B

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