434 Mr. R. Shida on the Number of Electrostatic 



(a) To determine a. The formula (2) also shows that, 

 whatever he the value of R, the product R tan a is a constant 

 quantity as long as E is kept constant; which furnishes this 

 important suggestion — that by varying the resistance R we 

 vary a, and thus get many values very nearly equal, if not 

 equal, of the product R tan a, the mean of which would be the 

 more accurate value of the product. The determination of a, 

 therefore, was performed as follows : — The current from the 

 gravity-cell was passed through the tangent-galvanometer g 

 and a variable resistance r, and the deflection a was noted. 

 The object of introducing the variable resistance is (1) to 

 enable us to alter the resistance R, and (2) to obtain the de- 

 flection giving minimum error, which is 45°. 



(b) To determine R( =g + b + r). The resistance g of the gal- 

 vanometer was measured by the Wheatstone's-bridge method, 

 and was equal to 30*86 ohms. The resistance b of the battery 

 was measured by measuring the deflections produced on the 

 scale of Sir William Thomson's quadrant-electrometer by con- 

 necting the electrodes of the cell to those of the electrometer, 

 first when the cell was unshunted, and secondly when it was 

 shunted by a known resistance. The resistance b in this case 

 is equal to the product of the difference of the two readings 

 into the shunt, divided by the second reading. It was exactly 

 equal to 2 '02 ohms. The corresponding values of a, r, R, so 

 obtained, were as follows: — 



45° 15' 80 ohms 107*88 



42° 45' 100 „ 112 



51° 39' 50 „ 82 



7-88) 

 2-88 } 

 2-88) 



.*. the mean value of R tan a = 104*73 x 10 9 . 



It must, however, be remembered that in all these measure- 

 ments the ohm, or B.A. unit of resistance, is assumed to be 

 exactly 10 9 C.G.S. units; which is unfortunately doubtful, as 

 was well remarked by Professor Adams, the President of this 

 Section, in his address. 



(c) To determine H. The method of determining this ele- 

 ment consisted in (1) observing the period of vibration of a 

 magnet under H, and (2) observing the deflection of a mag- 

 netometer placed in the magnetic meridian by the action of 

 the magnet placed at a fixed distance in a line at right angles 

 to the magnetic meridian and passing through the centre of 

 the magnetometer. I made the experiment with two different 

 magnets made out of very hard-tempered steel wire, about 

 0*97 centim. in diameter, and also experimented with each 



