96 Mr. W. Duddell: Jnstrumenss for the Measurement 
through the strip, then it heats and twists up, rotating the 
mirror M. Owing to the fineness of the strip (0°001” Pt Ag) 
which is heated by the current, the instrument is very quick 
in action. The mechanical periodic time is also very short, 
about ,; second ; so that it is able to follow with a fair 
accuracy currents which vary over a small range as rapidly 
as one or two cycles per second. The data of this actual 
instrument are :— 
Resistance 20 ohms. 
Current to give 25 cm. at 100 cm. scale - distance, 
22 x 10-* ampere. 
P.D. to give 25 cm. at 100 em. scale-distance, 0-44 volt. 
So that taking 1 cm. as the smallest measurable deflexion 
and 0:1 mm. asthe smallest detectable movement— 
The smallest measurable current is 4:4 x 10—-° ampere. 
- detectable : 0-5 x 10-° ampere. 
Watts required to produce smallest measurable deflexion, 
2. e. 1 em., 387 micro-watts. 
It is evident that this very simple instrument has many 
uses. It is very easily set up, requiring no careful levelling, 
and is quite robust. The present instrument which I made 
three years ago has often been carried about just as it is in 
the pocket. The self-induction of the wire is extremely small, 
and the temperature-coefficient of the resistance of the wire 
(“Pt Ag) is small also; so that the instrument can be used 
as a voltmeter to measure voltages down to about O01 volt, 
and ] have used it in series with high resistances to 
measure voltages up to 10,000 volts. The instrument can 
of course be shunted to measure large currents ; but if a good 
deflexion of say 25 cm. is required, the drop in volts on the 
shunt, 0-44 volt, is serious. Its main defect is that it requires 
screening trom quick vibrations of the order of ,!5 second, as 
the damping is not quite sufficient. I find that this can be 
easily done by standing the instrument on a heavy block which 
is suspended by means of wires and springs as in the Julius 
suspension for galvanometers. 
The chief practical use to which I have put the instrument 
has been to observe the quick variations of the R.M.S. 
voltage in supply-stations produced either by cyclic irregu- 
larity of the engine or by phase-swinging between alternators, 
converters, &c, or this purpose I put the instrument in the 
place of my oscillograph in the ordinary film-camera recording 
apparatus, and photograph the movement of the spot. 
It is easy by working to a false zero to obtain a sensibility 
of 1 em. change in deflexion for 1 per cent. change in the P.D. 
