604 Hot-Wire Ammeter for measuring Small Currents. 
and the deflexion of the hot-wire ammeter, were found to be 
as follows :— 
R=3450 ohms, V’=114°5 volts) V =2000s ales 
Hence the magnetizing current of the transformer was 
114°5/3450=0-033 ampere. 
The hot-wire ammeter was calibrated as above described, 
and the reading of the magnetizing current taken with it was 
found to be 0°034 of an ampere. Hence the value of the 
same current as determined by the electrostatic voltmeter, 
and the high resistance by the ratio V/R and that determined 
by the hot-wire instrument were in very fair agreement. 
The hot-wire ammeter has, however, the advantage over 
the high-resistance and electrostatic voltmeter method, that 
it 1s easier to calibrate and also involves a less drop in voltage, 
the more so as the current to be measured is smaller. 
This form of hot-wire ammeter has many uses. It can be 
employed as a relay operated by small alternating currents to 
set in action appliances only workable with continuous 
currents. In this case the sagging wire is made to drop a 
steel needle on toa mercury surtace and thus close the circuit 
required. 
As a research instrument, it may be useful because it 
appears that the deflexion of the ray of light created by the 
sag of the wire within useful limits of working is almost 
exactly proportional to the square of the current passing 
through the wire, and hence standardization is effected by 
passing one curr ent of known value through the instrument, 
whilst if a suitable wire is employed, it can “be calibrated with 
sufficient accuracy for most technical purposes by applying 
to the ends of the ammeter wire 2, 4, or 6 volts obtained from 
_1, 2, or 3 secondary cells, if need be a suitable resistance being 
interposed. For measuring alternating currents of a few milli- 
amperes for electromedical purposes, it should also be useful. 
When currents larger than a few milliamperes have to be 
measured, it can easily be accomplished by the same fine 
wire ammeter, by shunting the terminals with a resistance 
either equal to or one- half, one-third, or one-nth of that of 
the wire resistance. ‘Thus, if the above described ammeter is 
shunted with 168 ohms, or with 84 ohms, or 42 ohms, then 
the ammeter readings fee to be multiplied by 2, by 3, or 
by 5, to evaluate the ‘whole cur rent, and if shunted by a shunt 
of one-nth of its own resistance, then the readings have to be 
multiplied by n+1. 
The above described ammeter has been skilfully made for 
me by my assistant, Mr. A. Blok, who has also carried out 
for me the measurements described. 
