54 Mr. S. R. Milner and Prof. A. P. Chattock on 



between a and a' was also made through a platinoid wire from 

 a point on which another platinoid wire led to a second 

 binding-screw T close to R. T and R were thus the points 

 to which the galvanometer was connected, and they were 

 wrapped together in cotton-wool. 



The mean of many determinations gave for the temperature- 

 coefficient of the platinum heating-coils at 18° C, 



« 18 = 0-003330 



or referred to 0° C, 



« = 0-003530. 



The resistances of the inner portions (aft and a'fi') of the 

 heating-coils were 



R.H. («/3) =1-650 ohm at 18° C. 

 L.H. («'/3') = 1*656 „ „ 



Consequently the temperature-difference between the two 

 coils per cm. displacement on the bridge- wire was — 



For R.H. coil Leated 0-1192° C. 

 L.H. „ 0-1188 



Heating- Current. 



This was supplied by storage-cells working through rheostats, 

 and was measured by a gravity electrodynamometer specially 

 constructed for the work. Its constant, obtained in terms of 

 the Clark cell and resistance, was 0*04769; i. e. the current in 

 amperes = 0'04769 ^8, where 8 is the reading of the head of 

 a micrometer-screw divided into 100 parts. The indications 

 of this instrument could be relied on to less than 0'1 percent, 

 when measuring the - 5 ampere actually used. 



Calculation of Remits. 



In addition to the previous symbols on p. 47, let 



r be the thermal resistance per sq. cm. of the paraffin- 

 wax and glass between the heating-coil and the water ; 



/3 its coefficient of increase with temperature ; 



k' the thermal conductance per sq. cm. of the cotton- 

 wool between the heating- coil and the balancing-coil. 



A balance is first obtained on the bridge-wire with a current 

 C through the heating-coils and an unknown water thickness x. 

 If 6 is the temperature-difference between the heating-coil 

 and the sink of heat, and d that between the heating-coil and 



