84 Mr. W. Ellis Williams on the Bujrfure of 
tests. The cylinder being left open, the iron core was hung 
from one pan of a balance placed directly above the cylinder, 
and the force for each current measured by placing weights 
in the other pan. The values obtained are given in the 
following table : — 
Current 
j Force in 
Friction in 
1 Current. 
if orce. 
Friction. 
in amps. 
grammes. 
grammes. 

553 
2 
11-2 
2470 
5 
4-3 
870 
9 
13-0 
3120 
10 
5-53 
1042 
3 
14-8 
3570 
20 
6-7 
1282 
5 
15-7 
3700 
20 
8-16 
1670 
5 
16-0 
3920 
20 
9-03 
1905 
5 
It will be seen that the maximum force obtained was 
nearly 4 kilos, and that the friction never exceeded 1 per 
cent, of the total force. 
Preparation of the Rocksalt Pieces. 
Owing to the extreme brittleness of the rocksalt it was 
found necessary to suspend the pieces in such a way that there 
should be no bending moment on the piece, that is to say 
the line of resultant stress must pass through the centre of 
each cross section of the rocksalt rod. For this purpose the 
apparatus shown in fig. 2 was made. It consists of two 
cylindrical pieces made in aluminium, with holes 
of the proper size into which the rocksalt rods 
could be fastened with wax. Each piece has a 
short steel rod pointed at each end screwed in. 
The inner two of these points serve to suspend the 
piece in the testing apparatus, while the other two 
serve to mount the piece in the lathe during the 
preparation of the rocksalt rod. These steel rods 
were carefully fitted in on the lathe so that they 
were exactly parallel to the axes of the cylindrical 
pieces. The rocksalt was first split with a knife 
into prisms about 3 cms. long, and was then 
worked approximately cylindrical with a fine file. 
The aluminium end-pieces were then fixed on, 
and while the wax was still soft the whole was 
pushed into a brass tube into which they accurately 
fitted, and left in until the wax hardened. This ensured that 
the axes of the two end-pieces should be in the same straight 
