AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
G 5 
The last two experiments show that with copper the temporary alterations of 
resistance caused by longitudinal stress increase up to a certain limit of permanent 
extension and then begin to decrease. We also see that the greatest changes in the 
values of the temporary alterations of resistance take place at those places where 
there are the greatest permanent changes of resistance. 
The Effect of Compression on Electrical Conductivity. 
It was anticipated that compression would in most cases produce decrease of electrical 
resistance, and as carbon seemed a suitable substance to experiment on, the following 
arrangement was made with the view of ascertaining whether compression would 
produce on the resistance of carbon the opposite effect to that caused by longitudinal 
extension :—S in fig. 11 is a block of stone on which rests upright the piece of carbon 
Fig. 11. 
which it is desired to compress; the other piece of carbon to serve as the comparison- 
piece is supported near the first by a clip-stand (not shown in the figure). The two 
pieces of carbon were each about 4 inches in length, and were well coated with copper 
deposited by electrolysis for a distance of about three-quarters of an inch at the ends. 
To the upper ends a, a were soldered two silk-covered copper wires as in the experiments 
on extension, and the junction of these was connected with the galvanometer, whilst to 
the copper deposited on the lower extremities b, b were soldered wires to serve for 
connecting with the battery and resistance coils. The compression was produced by 
putting weights into a scale-pan attached to the end B of a lever of hard wood, which 
m the position of the carbon rod produced on the latter a pressure five times that of 
the weights in the pan. 
MDCCCLX XXIII. 
K 
