
A New Form of Sensitive Hot- Wire Voltmeter. - 371 
avoided by means of galvanometers and other, sometimes 
complicated, apparatus. Some of the lines of research in 
recent years have necessitated an extensive use of. quadrant 
electrometers, with the natural result that they have been 
greatly improved; and whatever reasons there may formerly 
have been for avoiding the use of electrometers, these reasons 
have now entirely disappeared. 

XLIV. On a New Form of Sensitive Hot-Wire Voltmeter. 
By R. THRELFALL, F.R.S.* + 
7PSHE practical need for a sensitive alternate-current volt- 
| meter arises in connexion with the measurement of large 
alternating currents. ‘The instruments at present employed 
for the purpose of measuring alternating currents are substan- 
tially of two types,—the ampere gauge of Lord Kelvin forming 
the standard and almost only representative of one class ; and 
instruments based on transformers forming the other. Both 
classes of instruments require calibration in manufacture, 
and from time to time, and it is then that the want of a 
sensitive voltmeter is felt; for the obviously most direct 
method is to measure the potential drop across a standard 
resistance traversed by the whole current in question. It 
may seem curious that the larger the current to be measured, 
the more sensitive must be the voltmeter employed ; but a 
little consideration will show that this is the case because it 
is not practicable to go on increasing the weight of a standard 
resistance without limit. In the other alternative the heating 
becomes excessive, and there is a risk of damaging the 
standard. for instance, suppose that it is a question of 
measuring 2000 amperes by means of a resistance of 
-0002 ohm, the P.D. drop is °4 volt, and the power expended 
in heat is *8 kilowatt—quite a consideration. The practical 
standard alternating-current voltmeter must therefore be 
sensitive and adjusted to work across an external resist- 
ance which may be considered negligible in comparison 
with its own resistance. Ifthe hot-wire form be adopted, it 
is seen that the electrical considerations point to the wire 
being as short as possible, and also as fine as possible ; for it 
has often been shown that the rise of temperature for a given 
current-density increases as the diameter of the wire decreases. 
Taking everything into account, the most suitable material 
appears to be pure silver. As this can be obtained com- 
mercially nicely gilded and of such a thickness that two miles 
* Communicated by the Physical Society; read November 27, 1903. 
