600 Prof. J. A. Fleming on a Hot-Wire Ammeter for 
of light on the scale placed at a distance of about 80 ems. 
from the mirror, produced by an application of two volts to 
the wire, is about 3 centimetres, and 4 volts produce about 
12 centimetres deflexion: hence, current of about 1/100 of 
an ampere or 10 milliamperes produces a deflexion which 
can be accurately read to within 2 or 3 per cent., and a 
current of as small as 5 milliamperes thus can be measured. 
The particular class of wire with which tbe instrument 
should be strung depends on the uses to which it is to be 
put. If the object is to read a current of as small a 
value as possible, then the wire must be as fine as possible 
and made of a material of high specific resistance, such as 
constantan. 
Messrs. Hartmann & Braun of Frankfurt have recently 
given attention to the production of very fine wires drawn from 
different pure metals and alioys, and they are able to furnish 
wires of pure metals and high-resistance alloys drawn down 
to diameters varying between 0°05 mm. and 0°02 mm. The 
resistance of a constantan wire of the latter size per metre is 
about 1350 ohms, whilst a wire of pure silver of the larger 
size has a resistance of only 8 ohms per metre. 
The sag of the wire used in the above described instru- 
ment depends essentially upon its temperature, and its 
temperature depends upon the rate at which energy is being 
expended in it, per unit of its surface. Accordingly, for the 
measurement of the smallest currents the wires must be of 
high-resistance material and as small as possible in diameter, 
whilst for the measurement of small voltages the wire must 
be made of a material like silver with high conductivity. 
The resistance R of the ammeter wire corresponding to 
different currents A through it can be determined as follows:— 
The ammeter wire is joined in series with a plug-resistance 
R,, and also with a constant resistance R, which may be 
either 20 or 40 ohms. 
Fig. 4. 
ll - - - 

To Pof= To Pol= 
The ammeter wire is also shunted by a divided resistance 
R’ (see fig. 4), and from a section of this resistance and from 
the terminals of the resistance R, wires are taken to a poten- 
tiometer. A battery of 100 volts is connected up, so as to 
