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• The following are the data of some Ayrton-Mather Galvanometers recently constructed : —

Resistance of coil in ohms. Period of coil in seconds out of magnetic field. Deflection in mm. at 1 metre Factor of Merit.

per micro- amp. pel-microvolt. per । micro- ’ coulomb.

7 3-0 18 2-6 37 90

7 8-0 139 19-8 110 100

20 8-2 245 12-2 190 110

20 3-5 53 2-6 100 130

146 7-8 540 3-7 435 123

400 8-3 757 ' 19 590 100

Ayrton-Perry-Duddell Twisted Strip Galvanometer.

(1) “ Instruments for the Measurements of Large and Small Alternating Currents,” by W. Duddell, Phil. Mag., July 1904.

^2; “ LTndustria Rivista Tecnica,” May 6th, 1906.

The working portion of this galvanometer consists of a fine metal strip twisted after the manner suggested by Ayrton and Perry. Half the length is twisted in one direction, and the other half in the opposite direction.

In the centre of this strip are fixed a mirror and a mica damping vane. The strip, together with two straight wires of the same metal, is stretched between two blocks by means of a spring.

Any change in the atmospheric temperature will tend to lengthen or shorten equally the strip and wires, and so the mirror does not rotate. If a current, however, is sent through the strip, then it heats and twists up, causing tlie mirror to rotate.

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