Old Prof. R. Threlfall on a New Form of 
The above principles are put into practice in the following 
manner. The active wire is carried between two adjustable 
supports, one of which must be insulated from the rest of the 
apparatus. The other support can be moved to and fro in 
the direction of the wire by means of a micrometer-screw, 
and the motion may be further reduced by means of a lever, 
if so desired. The mirror may be a centimetre long and 
4 mms. wide, and the hinge-wire may be about 2 mms. from 
the vertical plane of the active wire. The mirror is backed 
by a scrap of mica, and is supported by the head of the hook 
which pulls the active wire down. As the active wire heats 
up, the hook descends and the mirror is tilted downwards, 
There is a small incandescent lamp in the lid of the box 
enclosing the instrument, and a ground-glass or celluloid 
scale opposite a slot in the front of the case. By means of a 
prism and lens an image of the filament is thrown on the 
screen by the mirror, and by turning the micrometer-screw 
this image can be brought to any desired point. The mirror 
is not affected as to position by swinging the box from the 
end of a rope; butitis sensitive to vibrations of short period, 
and it is best to observe when the instrument is suspended 
rather than when it is resting on a table—especially in an 
engine-house. Small voltages can be read off at once on the 
scale, but larger ones are compensated by working the screw. 
The whole apparatus is enclosed in an aluminium box mea- 
suring 1 foot x 1 foot x 8 inches high ; and these dimensions 
can be reduced if desirable. 
The following data refer to an instrument designed to work 
with potential-differences up to about 0°5 volt:— 
Distance between supports ...... 6°) cms. 
Titra! sao: ot wie 16) eae LOT ge 
Interiomangle aim ue aes 175° 30/ 
Resistance cold with leads about 1:2 ohms. “fae 
Effective distance, mirror to screen in cover of instrument, 
28 ems. 
With this instrument ‘03 volt gave a deflexion which could 
just be easily seen without the assistance of a lens, and 
required 54, turn of the micrometer-head to compensate it. 
As it was intended to employ the instrument in measuring 
large alternating currents, 1t was thought well to calibrate it 
by sending a unidirectional current through the standard 
manganin resistance of ‘0002 ohm which it was proposed to 
employ ; and at the same time measuring the P.D. across this 
by a potentiometer and cadmium-cell. 
Four settings of the micrometer-head were made for each 
