Materials under Combined St 
ress. 
83 
pull on an iron rod placed partly inside a coil carrying an 
-electric current. The actual arrangement of the apparatus 
is shown in the accompanying figure. In order 
that the force might be as great as possible two 
coils were used, wouud on the same brass tube 
and about 15 cms. apart. This tube served also 
to guide the iron cores, which were connected 
together so that they moved as one rigid piece, 
the distance between them being the same as 
that between the coils. In order to diminish 
the friction between the cores and the guiding- 
tube,, a small frame carrying three rollers was 
fixed to each core. In the part of the brass 
tube above the upper coil a long slit was cut 
reaching to within 2 cms. of the top of the coil. 
A piece of thick steel wire was soldered to the 
upper end of the iron core and bent so as to 
travel in this slit, it thus prevented the iron core 
from turning, and at the same time served as a 
hook by which the core could be hung from the 
piece to be tested. The other end of the test- 
piece was supported on a ring-nut travelling on 
the brass tube on which a corresponding screw 
was cut. By adjusting this ring the core could 
always be suspended in the same position in the 
tube although the test-pieces might vary in 
length. In order to show when the test-piece 
was broken, it was arranged that the iron core 
in falling should break the electric circuit. 
For this purpose a ring carrying a small 
ebonite mercury cup connected by a piece of 
flexible wire to the upper coil was made to slide 
on the tube below the hook mentioned above. 
A wire coming from the cylinder cover dipped 
into this cup and closed the electric circuit of 
the two coils. When the test-piece was broken 
the steel hook in falling carried the ring with 
it and so broke the connexion. In the experi- 
ments the current was gradually increased until 
the connexion was broken and the maximum 
reading of the amperemeter noted. 
The connexion between the force exerted by 
the iron cores and the current in the coils w T as 
measured in a preliminary experiment. The 
brass tube carrying the coils was placed in the 
cvlinder in the position which it occupied in the breaking 
G 2 
