of Large and Small Alternating Currents. 98 
at a deflexion of 400 divisions, in order that the corrected 
deflexion might be accurately proportional to the square of 
the current flowing. 
To give an idea of the time the instrument requires to 
take up the permanent deflexion corresponding with any 
current, Table I. is given below of the deflexions at different 
times after starting a current of 8x 10-4 ampere; and after 
interrupting the same, to show the time required for the 
instrument to return to Zero. 
From which it will be seen that the instrument attains 
after 10 seconds its full deflexion to within 1 part in 500, or 
as it isa square law instrument indicates the true current to 
within 0°1 per cent. 
Taste I. 
| 
| ae seconds Time in seconds | 
after starting a Deflexion in | foe iitereaat | Deflexion in | 
current of seale-divisions. | i mnverruphns | scale-divisions. | 
8x10-4 ampere. | sie maa 
5 500°5 5 ae Caey, 
10 501-0 10 | 1:0 / 
30 501°5 30 | 0°5 / 
60 502'0 60 | 0} | 
120 502'0 120 | 0 
180 502-0 180 | 0 
| 
This instrument, which forms practically a_ sensitive 
alternate-current galvanometer, worked very satisfactoriiy, 
even with the highest frequencies used of 120 000 ~ per 
second. As examples of its sensibility, it may be mentioned 
that on making a suitable noise into a Bell telephone-receiver 
sufficient current is generated to send the spot off the scale ; 
and that if the thermogalvanometer be connected to the line 
wires of a microphone-tra ansmitter arranged in the ordinary way, 
whistling at a distance of from 15 to 20 feet from the micro- 
phone will cause deflexions of the instrument of several 
hundred scale-divisions. As the instrument will give a de- 
flexion of over two scale-divisions for one millivolt alternating, 
it might in some cases replace, with advantage, the telephone 
in the Kohlrausch method of measuring the resistance of 
electrolytes, as it would avoid the difficulties attendant on 
estimating the position of a sound minimum. 
It is perhaps of interest to compare the current taken by 
the thermogalvanometer with that which flows through an 
electrostatic voltmeter when both are used to measure the 
|S. 
ed 
