98 Mr. W. Duddell: Jnstruments for the Measurement 
in fig. 4, and a photo of the actual instrument with the case 
removed in fig. 5 (Pl. III.). 
In the field between the pole- Fig. 4. 
pieces NS (fig. 4) of a permanent Tr 
magnet is suspended by means of a 
quartz fibre a single-turn coil or loop 
of wire /, to the lower ends of which 
is fixed the thermocouple Bi, Sb. 
This loop is surmounted by a glass 
stem g anda mirror M. Below the 
lower junction of the thermocouple 
is fixed the heating resistance h, one 
end of which was connected to the 
frame of the instrument to avoid 
electrostatic forces. The action of 
the instrument is as follows. The 
current to be measured flows through 
hand raises its temperature, causing 
the lower junction of the thermo- 
couple to rise in temperature above 
the upper, thus producing a current 
round the loop, which is deflected by 
the magnetic field against the tor- 
sion of the quartz fibre. 
The data of the instrument are:— 
Resistance 18 ohms. 
Current to give 25 cm. at 100 cm. 
scale-distance, 8 x 10-* ampere. 
P.D. to give 25 cm. at 100 cm. 
scale-distance, 14°4 x 10- volt. 
So that, taking as before 1 cm. as 
the smallest measurable and 0:1 mm. 
as the smallest detectable deflexion, 
The smallest measurable current 
is 1°6 X 10~* ampere. 
The smallest detectable current 
is 0:2 x 10-* ampere. 
Watts required to _ produce 
smallest measurable  deflexion, Bi lil Sb 
‘0°46 micro-watt. =\h 
The deflexions of the instrument 
were nearly proportional to the 
square of the current ; assuming the 
instrument correct at 100 divisions 
deflexion, then higher deflexions required to be increased by 
amounts gradually increasing up to between 4 and 5 divisions 
