282 Mr. P. E.Shaw: 



This rough value of /, though small, is not small enough, 

 for I have frequently examined the surfaces after cohesion 

 with a microscope and haA-e never seen any markings even 

 with high -power objectives. The bridge is assumed to "be all 

 over the area of contact, ird 1 . If we assume that the bridge 



is formed over -th of the area of contact, I would be -th as 

 much . 



Knowing the area of cross-section of the bridge and the 

 force required to break it, i. e. to sunder the wires, we can 

 determine the breaking-stress (F) per unit area of the 

 material. 



Let the force be 1 dyne, then 



F= . * r =2xlO»O.Q.ft; 



5 x 10 J 



but this is about the breaking-stress per unit area for copper, so 

 that there is good agreement between theory and experiment. 



When current passes through the contact, it is obvious 

 that melting will occur over a very small area — but as sjoii 

 as any melting occurs, the area in contact rapidly increases, 

 and the conditions are changed. 



Consider the case of a current =0*001 a and resistance 

 ^=10 ft). 



Suppose the copper acted on has an area 10~ 6 sq. cm. and 

 a depth 10~ 6 cm. Then, if the density is and specitic 

 heat 0*1, the rise in temperature in one second- 



G 2 .R 1 



X 



4-2 M . K 

 (0-001) a xl0 1 



4-2 10- 6 xlO- 6 xUxO*l 



= 2x10* about. 



Suppose only -y ¥ Vo °f ^ ne nea * produced is effective, we 

 should have a rise in temperature in one second of 2000° 

 about. This would be ample to weld the surfaces under the 

 pressure existing, which has been already shown to be of the 

 order 200 atmos. 



If the results obtained above are rough, it must be re- 

 membered that considerable conjecture has had to be made ; 



* The latter would probably be the case. If - = Trrr^f 1=3x10— 5 f 

 which is a more resisonable result. n 



