Investigation of the Simple Coherer. 269 



where, in our case, 



v-v- i — i 



i 



" x " 2 radius of cylinders 

 The elastic constants are expressed in KirchhofFs notation : 



yu,=Poisson's ratio, 



E = Young's modulus. 

 We can take for our purposes 



E = 10 12 c.g.s., 

 and let v=\ mm., then p = 20. 



Take two cases : — 



(1) Suppose p = l dyne: 

 working out I. and II., we have 



a=4'lxl0- 5 cm. 

 a=3-25xl0- 8 cm. 

 (2) Suppose p= 100 dynes, then 

 a=l*9xl0- 4 cm., 

 «=7-0xl0- 7 cm. 



These values of a and a enable us to ascertain in any 

 special case (1) the area of contact of the wires, and (2) the 

 amount of compression at the place of contact. 



It will be seen that in the above calculations p has been 

 given extreme values, 1 dyne and 100 dynes. For present 

 purposes only the order of p is required ; it was found as 

 follows: — 



At right angles to the moving wire (ab in fig. 1) was 

 attached a fine silk fibre passing over a very small light 

 watch-pulley s (fig. 2) and then vertically down to a small 

 weight m. The tendency of the weight is to pull the wire 

 ab round towards the pulley; this can be counteracted by 

 putting on sufficient current in the galvanometer circuit. 



There are two cases : — 



(1) When the weight was attached : 



It was found that when a current 0*00125 a. was put 



