312 Prof. R. Threlfall on a New Form of | 
go to the tr oy ounce, it was not worth while trying to 
improve upon it in the laboratory, though no doubt finer stuff 
could be produced by special artifice. For instance, by 
making a silver wire or strip the anode ina dilute solution 
of potassium iodide, it can be very materially reduced in 
thickness, but the mechanical properties are impaired to some 
extent in spite of the equalizing action of the coating of silver 
iodide which is formed. 
In order to make use of the gilt silver wire as a hot-wire 
voltmeter, it is necessary to be certain that the wire is always 
stretched by the same force. and then the small changes in 
length consequent on the passing of a current can be measured. 
The chief peculiarity ot the instrument in question is in 
regard to the means adopted for securing uniformity of. tenaion 
of the fine wire. 
Referring to fig. 1. The fine wire is seen stretched baba 
two supports, and pulled downwards at the centre by a micro- 
scopic hook and spiral spring insulated from the base; a small 

‘mirror hinged on a wire stretched alongside rests on the head 
of the hook. The deflexion of the wire is exa ggerated i in the 
diagram, in reality the interior angle is about 175°, or from 
that to 178°. 
