DETERMINATION OF THE SPECIFIC HEAT OF WATER. 451 
magnetism, we took the precaution of calibrating the instrument immediately after 
each equivalent experiment. This was done as follows :— 
The current from two storage cells passed in succession through the current meter, 
a coil of manganin wire and a set of adjustable resistances (see fig. 6), The 
terminals of the manganin coil were connected up through the astatic galvanometer 
with our Standard Clark cell, so as to oppose the E.M.F. at the terminals of the 
manganin coil due to the passage of the current from the storage cells to the E.M.F. 
of the Clark cell. If the galvanometer shows no deflection on both circuits being 
closed, then e = zr, where e = E.M.F. of the Clark, 7 = current in the current meter, 
7 = the resistance of the manganin coil. 
A coil of manganin wire was chosen, since this material has a very small tem- 
perature coefficient ; it was supplied by Messrs. Exxiorr, having a composition of 
copper 84 per cent., manganese 12 per cent., nickel 4 per cent.; its resistance value 
was nearly 14 true ohms. An example will explain the calculation of the value of the 
current. On May 20 the current meter read 48°34 divisions (a short focus lens 
enabled us to read the divisions to hundredths). Temperature of the Clark 
cell 17°12 C. Hence the current is equal to 1°4827/14 = 0°10234 ampére; and 
therefore 1 division = 0°00212 ampére. The instrument remained very constant 
throughout the equivalent experiments, as may be seen from the following table :— 
TABLE IX. 

Wate Value in amperes of 
; 1 division. 

WER ANON Gs We Sg. es 0:00213 
ode SSD W eS 2, See HERP ee 0:00212 
© ey slideu aren ptpascme S 000212 
HD) ( nse code oes 0:00212 
afin)" § Wie) 6. oboe 000212 
EGP Ko) hehe eae, Bene 0:00212 
SLi Lis f ives <a Soar nalce 0:00212 



As a check upon this method of calibrating the current meter, it was also 
calibrated on one occasion by measuring the current passing through it by the 
silver voltameter. Its constant as thus determined was 0°00212. The sensibility of 
the current meter was such that its readings were accurate to 1 in 400. Hence the 
cooling correction to be applied is accurate to 1 in 200, and from the data already 
given it appears that an error of that amount only affects the accuracy of the 
equivalent to 1 in 200,000. Knowing the value of the auxiliary current, the rise 
in temperature (#) per minute of the total mass of water is calculated from the 
formula 6 = (C°R x 60)/(J x M), R being the resistance of the heating coil. 
The mean of several determinations of R with an Elliott P.O. box—which had been 
3M 2 
