Sensitive Hot-Wire Voltmeter. 379 
observation, and the mean taken. It was found that the zero 
reading varied by two or three twenty-fifths of a turn as the 
temperature changed ; also it required about half a minute for 
the wire to reach its steady state after the voltage was applied. 
In all cases the micrometer-screw was brought to near its zero 
reading before the current was interrupted, though a special 
experiment had shown that no damage appeared to result from 
the neglect of this precaution. 
Curve of Calibration of Hot-Wire Instrument. 
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| : 
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200 
HtAo PeAoines 
Pe 
100 



500 1000 1500 2000 2500 
CURRENT 1 STANDARD FESISTANCE 
The micrometer-head was divided into 25 large divisions, 
and each large division was subdivided into ten small divisions. 
The unit in the column “ Head Readings ” is one large division 
of the head. 
The source of light was a 4-volt } C.P. lamp in the top of 
the case fed by two pocket storage-cells, 

