374 Mr. H. Tomlinson on the Temperature at which 



first, except that they had no pipeclay coating and no nickel 

 core inside, was kept buried in a box filled with well-dried 

 sawdust and placed about two feet away from the air-chamber. 

 The two primary coils were connected up in series with each 

 other, a battery, a key, a rheostat, and a tangent-galvanometer 

 whose constant had been previously carefully determined. 

 The secondary coils were also connected up in series with 

 each other, with a rheostat, a Thomson's reflecting-galvano- 

 meter, and an earth-coil. The earth-coil consisted of seven 

 turns of silk-covered copper wire laid side by side round the 

 circumference of a wooden disk 30 centim. in diameter ; the 

 disk was supported with its plane horizontal, but could be 

 turned round a horizontal axis in either direction through an 

 angle of 180 degrees. The ends of the wires round the disk 

 were soldered to amalgamated copper disks revolving in 

 mercury-cups, which last served to connect the earth-coil 

 with the rest of the apparatus in series with it. The two 

 pairs of primary and secondary coils were so arranged that 

 the currents induced on opening or closing the battery-circuit 

 by means of the key exactly balanced each other before the 

 introduction of the nickel into one of them. When therefore 

 the nickel was introduced, the observed deflection produced 

 by opening or closing the primary circuit was entirely due to 

 the magnetic permeability of the nickel ; and by comparing 

 this deflection with that produced by turning the earth-coil 

 suddenly through 180° the permeability could be determined 

 in absolute measure, since the vertical component of the 

 earth's magnetic force at the place was known. The earth- 

 coil also served another purpose, namely to secure with the 

 aid of the rheostat the constancy of the sensitiveness of the 

 Thomson-galvanometer. As the temperature of the air- 

 chamber was raised, the total resistance in both primary and 

 secondary circuits increased ; but by altering the rheostats in 

 these circuits until the deflection of the tangent-galvanometer 

 in the one case, and of the Thomson-galvanometer, when the 

 earth-coil was suddenly turned, in the other, became the same 

 as before the heating, constancy both in the magnetizing force 

 and in the sensitiveness of the Thomson-galvanometer was 

 secured. 



The mode of proceeding was as follows : — By means of the 

 set of burners underneath the air-chamber the temperature 

 was raised to nearly the highest point which it was desirable 

 to attain ; the burners were then adjusted until the tempera- 

 ture was either constant or very slowly rising or falling ; this 

 could always be secured by waiting tor a sufficient length of 

 time. The rheostat in the secondary circuit was then adjusted 



