﻿298 Lord Rayleigh on a Quantitative 



We have now further to imagine the second part of the 

 divided cylinder to be replaced by the plate of the telephone, 

 and that not in actual contact with the remaining cylindrical 

 part. The reduction of effect on this account I estimate at 

 4 times *. The force on the telephone-plate per unit area of 

 core is thus 



2000x47rwy; (8) 



or if, as for the telephone of 100-ohms resistance, r* = 2200, 

 and area of section = *31 sq. cm., 



force = l'7xl0 7 7 (9) 



In (9) the force is in dynes, and the current 7 is in C.G.S. 

 measure. If T denote the current reckoned in amperes, 



force = 1-7 xl0 6 T, (10) 



and this must be supposed to be operative at the centre of the 

 plate. 



We shall presently consider what effect such a force may 

 be expected to produce ; but before proceeding to this I may 

 record the result of some experiments directed to check the 

 applicability of (10), and made subsequently to the theoretical 

 estimates. A Bell telephone, similar to T 1? was mounted 

 vertically, mouth downwards, having attached to the centre 

 of its plate a slender strip of glass. This strip was also 

 vertical and carried at its lower end a small scale-pan. The 

 whole weight of the attachments was only "44 gram. The 

 movement of the glass strip in the direction of its length was 

 observed through a reading-microscope focused upon acci- 

 dental markings. The telephone, itself of 70-ohms resistance, 

 was connected through a reversing-key with a Daniell cell 

 and with an external resistance varied from time to time. In 

 taking an observation the current was first sent in such a 

 direction as to depress the plate, and the web was adjusted 

 upon the mark. The current was then reversed, by which 

 the plate was drawn up, but by addition of weights in the 

 pan it was brought back again to the same position as before. 

 The force due to the current is thus measured by the half of 

 the weight applied. 



The results were as follows : — 



External resistance in ohms . . 100 200 500 

 Weight in grams 8-4 2 



When 1000 ohms were included, the displacement on reversal 



* I should say that these estimates were all made in ignorance of the 

 result to which they would lead. 



