56 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
order that measurements of diameters may be made while a current is passing 
through the coils. The current used in our absolute measurements of a resistance 
did not exceed 2 amperes, and we experimented with such a current through the 
coils during our measurements of the diameters. Preliminary observations showed 
that the expansion of a cylinder 5 minutes after switching on the current was 
sufficiently great to allow of a definite measurement being made, and a thermometer 
recorded an increase of temperature of a whole degree in this time. A survey of the 
surface of the coil showed the expansion to be greatest in the mean diametral plane, 
and least at the ends ; measurements were, therefore, made on four turns of wire, 
these being the 1st, 16th, 48 th, and 96th strands measured from one end. 
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Fig. 10. Showing increase in temperature of marble cylinder and increase in diameter of certain parts of 
the coils Avhen a current of 2 amperes is passed through them for 37 minutes. 
O indicates temperature observations 
x ,, observations on strand No. 96 
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48 ► 
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measured from one end of coil. 
The results of the measurements are given in figs. 10 and 11. In fig. 11 the 
expansion of the coil is given 5, 10, 15, 20, 25, and 35 minutes after a current of 
2 amperes was switched on. The maximum increase in diameter is 10'4/x. Fig. 10 
shows the rise in temperature recorded by the thermometer when the current was 
left on for 37 minutes, and shows also the fall in temperature after the circuit was 
broken. A relation between the mean rise in temperature due to the current and 
