304 PROCEEDINGS OF THE AMERICAN ACADEMY 



The volume being unchanged by the stretching 



V = nrH 



From this we may conclude, that, if the original resistance is to the 

 final resistance in a greater or less ratio than the square of the original 

 length is to the square of the final length, the stress of the particles 

 does alter the specific resistance of the wire. If the ratio of the re- 

 sistances is greater than the ratio of the squai'es of the lengths, then 

 the conductivity of the wire is improved. 



In the following experiments I used a Thomson's mirror galva- 

 nometer with the arrangement of Wheatstone's bridixe. I used a 

 wooden bracket attached to a partition of the wall, about 2.5 metres 

 from the floor. I put the wire through a hole in this bracket, and kept 

 it from slipping by driving in a wooden peg. To guard further against 

 slipping, the wire was wound tightly around a screw near the hole. 

 Altogether there was from four to ten centimetres of wire that was 

 not stretched, but whose resistance was taken account of. In the later 

 experiments, this length was taken account of in calculating the re- 

 sistances. To the lower end of the wire I bound a ring, and on this 

 ring were hung the weights. I connected the ends of the wire with the 

 box of resistance coils by means of thick wires. The resistance of 

 these thick wires was found to be .036 ohms. 



Experiment 1. — This experiment was made with copper wire .628 

 millimetres in diameter. It being the first experiment I made, all the 

 phenomena were not observed. The original and final resistances, and 

 the original and final lengths, I got pretty carefully ; but I failed to note 

 the intermediate lengths. 



Original length was . . . .' 1.647 metres. 



Final length before breaking 1.857 „ 



Stretched 21 centimetres. 



Ohms. 



Original resistance with connections . • 144 



Kesistance of connecting wires 036 



Original resistance 108 



Final resistance without connecting wires 134 



