OF ARTS AND SCIENCES. 305 



We should expect B^ to have been .137. The wire broke under the 

 weight of 16,3 lbs. The weights I used were in lbs., not grammes. 



But we have not considered the whole length in considerina the 

 change in length ; but we have considered the whole length in consider- 

 ing the change in resistance. If we afld the same constant to / and I^, 

 say a, and /^ is greater than /, then plainly 



li' - d + «)■-■ 



But more than a mavhave been added to I,. 



Then, too, if the whole lenjjth had been under the stretchinn; 

 process, J^^ might have been greater than it was observed to be. So 

 that the difference of .003 olims might have been made up, had the 

 whole length been under the stretching process. 



In this experiment I measured the final diameter of the wire with 

 the dividing engine, and found that it varied perceptibly in different 

 parts ; in one part the mean reading being .593 millimetres, and in 

 another part .50 millimetres. From the deduction given above, it can 

 be seen that we need only consider the lengths and the squares of the 

 lengths. In the following experiments I did not consider the change in 

 diameters. 



Experiment 2.' — In this experiment I used thin iron wire. 4, 6, 8, 

 and 10 lbs. produced no change in the resistance of the wire, though 

 the length increased slightly. Original resistance was 1.0536 ohms. 

 At first, R was only 1.0584. 



10 mm. i? = 1.068 ohms. 



15 „ R = 1.0704 „ 



20 „ 7^ = 1.1256 „ 



25 „ 7? =1.1304 „ 



28 „ i? = 1.1404 „ 



30 „ i? = 1.1472 „ 



I did not ob.serve the changes in length carefully. At tliis time, 

 2 lbs. was the smallest M'eight I had. On applying fourteen pounds, 

 the wire stretched some ; but, when I allowed tiie whole force to come 

 on, the wire snapped near a place where it was wound. I applied the 

 weight again, and the wire snapped near the middle. 



VOL. XI. (N. S. III.) 20 



