450 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
The sum of the various small corrections to the water-equivalent amounts to 0°33, 
or about 2 parts in 10,000 of the whole water-equivalent. 
The Auxiliary Current. 
In the first two series of our experiments, the thermometer was generally rising 
during the first, but falling during the last period. It is known that a falling 
thermometer will somewhat lag behind the real temperature, but we had hoped to 
eliminate this by applying a constant correction. Special experiments made for the 
purpose did not, however, give us sufficiently consistent results, and it appeared that 
the lag was too variable to be allowed for. Hence we decided to work only with a 
rising thermometer. We might have achieved this by raising sufficiently the tem- 
perature of the surrounding water-jacket, but this would have made the gain by 
radiation and conduction in the first period greater than we thought could be correctly 
measured. The plan we adopted consisted in sending through our coil, in the final 
period, a weak current (about 0°1 ampere) which just overbalanced the cooling. The 
current—which we call the auxiliary current—could be measured with sufticient 
accuracy to calculate the rate of heating which was due to it alone, and as the rise in 
temperature was observed, the loss by radiation could be deduced. 
Fig. 6. 
resistances. 
Astatic 

galvanometer. 
The auxiliary current was derived from two storage cells and was measured by one 
of Lord KeExvin’s magnetostatic centiamperemeter balances placed in circuit with 
the coil and cells. In order to avoid any error that might arise from a possible 
change in the constant of the instrument due to a change in the value of the earth’s 
