OF ARTS AND SCIENCES. 305 



^ __ 2.71S _ Jl 108 R P 



/,- — ";5.45 — -'^6 7^ — 134 — -^^^^ IT^ > V 



AVe .sliduKl expect R^ to have been .137. Tlic wire broke under the 

 weight of 16.3 lbs. The weights I used were in lbs., not grannnes. 



But we liave not considered the whole length in considering the 

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

 ing the change in resistance. If we add the same constant to / and /j, 

 say a, and /j is greater ihan /, then plainly 



/2 (/ + «)2 



But more th;in a may have been added to /[. 



Then, too. if the whole length had been under the stretching 

 process, 7i*, might have been greater than it was observed to be. So 

 that the ditfei-ence of .003 ohms 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. 



Exppi-iment 2: — In this experiment I used thin iron wire. 4, 6, 8, 

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

 the length increased slightly. Original resistance was 1.053G ohms. 

 At first, i? was only 1.0584. 



10 mm. 7? = 1.068 ohms. 



15 „ li = 1.0704 „ 



20 „ li = 1.1256 „ 



25 „ A' = 1.1304 „ 



28 „ 7^ = 1.1404 „ 



30 „ A' =1.1472 „ 



T did not observe the changes in length carefully. At tliis time, 

 2 lbs. was the smallest weight I had. On applying fourteen pounds, 

 the wire stretched some; but, when I allowed the whole foi'ce 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. hi.) 20 



