270 



Mr. F. E. Shaw : 



on, the wire a b was brought into bare contact with the 

 wire cb. 

 (2) When the weight was not attached : 



Then the wires were brought together with a current 

 0-00119*. 

 Thus a current of 0*00006 a produces the same couple on 

 the system as the load (0*0031 gm.) acting at the contact 

 point. For future reference we may say that a current 

 6 x 10" 5 ampere produces at the contact place a pull or 

 push, according as it is put off or on, of 3 dynes. 



Fig. 2. 



ab 



ft 



Within certain limits, proportionality exists between 

 current and force; and we can lhake calculations on that 

 basis. 



This determination of pressure is admittedly rough, but as 

 only the order of the forces concerned is required, it is good 

 enough. In almost all the experimental work following, the 

 force used was of the order 1 dyne * ; very rarely it was 

 pushed up to the order 100 dynes, hence the limits used for p 

 in the above evaluation of a and a. 



It may be noted in passing that this is equivalent to an 

 average pressure over the area of contact of 200 atmos and 

 a maximum pressure of 300 atmos at the centre. 



It might be supposed that, when the wires are pressed 

 together gently as indicated, a current would necessarily flow 

 in the contact circuit; but, as the Tables following will show, 

 no current, or an imperceptible one, passes as a rule unless 

 some sort of coherence is definitely established, and that then 



* It will be seen that the atmospheric pressure on the pressure area from 

 which all the air is supposed squeezed out, is only nary. 10 6 = O005 

 dyne, which is negligible. 



