Properties of the Alloys of Iron and Aluminium. 125 



pieces a, #, were connected to the two terminals of the battery 

 B, vide figs. 1 and 2, and the two terminals b, b, to the points 

 y, 8, in the primary circuit (fig. 1). It will be seen that at 

 each half revolution of the secohmmeter the current in the 

 primary circuit was reversed. 



The contact pieces c, c were connected to the two terminals 

 of the galvanometer G (figs. 1 and 3), and d, d to the points 

 a, [3 in the secondary circuit. The secohmmeter was driven 

 by a small motor, and the contacts were arranged so that the 

 galvanometer connexions were reversed immediately before 

 the reversal of the battary. The direction of rotation is shown 

 by the arrow (fig. 2) . 



Pieces of ebonite were fitted into the spaces g g to allow 

 the central contacts to pass smoothly over them. In time the 

 insulation becomes impaired owing to the continuous deposi- 

 tion of brass on the ebonite. To prevent this, V-shaped 

 grooves h, h were cut in the ebonite, and thus the continuity 

 of the deposit was broken. These grooves were scraped out 

 from time to time during the experiments. 



Fio-. 3. 



The Branch Circuit. — To prevent the heating of the primary 

 coils due to the passage of the current for any lengthened 

 period a branch circuit Z (fig. 3) , whose resistance was equal 

 to that of the primary coils, was made use of. 



