of Metal Rods with rounded ends. 271 
tion, so that the release is extremely clean. Two points are of 
importance in using such an arrangement. First, the cord must 
be as inelastic as possible, for the time which elapses before the 
tension in the cord falls from its initial value to zero, is the time 
required by the dropper to gain on the rod a distance equal to the 
stretch of the cord under the original tension. Originally some of 
the fishing line used for suspending the rods was also used for this 
purpose. This line is very inelastic, but it was found when dealing 
with lighter rods (particularly aluminium) that this was more than 
counterbalanced by its rigidity, and it was replaced in the later 
experiments by strong cotton, which answered very well. Secondly, 
during the very short time that this change of tension inevitably 
occupies, the cord remains in contact with the pulley, and imparts 
a certain amount of spin to it. In order to make the energy thus 
lost as small as possible, it is necessary to keep down the friction 
and moment of inertia of the pulley. The pulley used was for this 
reason made of boxwood mounted on a light steel shaft which ran 
between two fine pivots at its ends. The release then proved 
most reliable and easy to manipulate. The dropper passes through 
a hole in the rubber block ( H , Fig. 2 A) on which it is caught by 
means of the collar, K, and so prevented from falling on to the 
pulley. 
The electric leads to the rods were made of fine silk-covered 
copper wire, formed into a helix top and bottom so as to be 
perfectly flexible (w lf w. 2) Fig. 2 A). They were attached to the 
rods by pushing a small helical spring ( B , Fig. 3) over a pin 
in the end of the rod. This formed a very light and good connec- 
tion. A spring catch (Fig. 6) was provided, which, after the first 
impact, caught the pin in the rod, r 2 , and so prevented a second 
impact from occurring while the galvanometer was being read. 
The sketch explains itself. 
We now come to the actual testing of the apparatus. The 
complete connections of the system are shown in Fig. 1 A, where 
Fig. l a. 
