TKANSACTIONS OF SECTION A. 499 



vary a aud lluis get many values, very nearly equal, if uot equal, of the product 

 R tan a, the mean of wliicli 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 (/ and a variable re- 

 sistance 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 

 deflection giving minimum error, which is 45°. 



(b) To determine 'R{= r/ + b + r). The resistance ff of the galvanometer was 

 measured by Wheatstone's bridge-method, aud was equal to 30-86 ohms. The 

 resistance b of the battery was measured by measuring the deflections produced 

 on the scale of Sir Wm. Thomson's Quadrant Electrometer by connecting 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 dift'erenee of the two readings into the shunt 

 divided by the second reading. It was exactly equal to 2-02 ohms. So that we have 



a r n 



45° -15' . 80 ohms . 107-88"] 



42° - 45' . 100 „ , 112-88 [ . •. the mean value of R tan a = 104-73 x 10». 



51° - 39' . 50 „ . 82-88 J 



It must, however, be remembered that in all these measurements the ohm, 

 or B.A. unit of resistance, is assumed to be exactly 10' 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 metliod of determining this element consisted in (1) 

 observing the period of vibration of a magnet under II ; and (2), observing the 

 deflection of a magnetometer placed in the magnetic meridian by the action ol" 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. "l made the experiment with 

 twodiftereut magnets made out of very hard tempered steel wire about 0-97 cm. 

 in diameter, and also experimented with each magnet by varying the distance of 

 the magnet, and found the results to agree very closely with one another. The 

 mean value of H obtained with one magnet is -15955, and the mean value obtained 

 with the other is -15937, so that the mean of these two is 



H = -16947 



The formula used in the calculation of H is 



H 



where t is period of vibration of magnet under H ; k distance of the centre of the 

 magnet from the magnetometer ; I half the length of magnet ; i the moment of 

 inertia of the magnet ; (j) the angle of deflection of the magnetometer. 

 We have now come to the evaluation of ' v.' The formula (II.) gives 



e = 1-01172 X lOs (0. G. S.) electro-magnetic units. 

 Hence 



V = 294-4 X 10^ centimetres per second, 



which agrees well with the latest value obtained by Sir Wm. Thomson, namelv, 

 293- X 108. > J} 



Although I took as much care as possible in making all the above measurements 

 leading to this evaluation of ' r,' yet since, from want oi time, it was only on one 

 occasion that I was able to make the complete measurements, there may have 

 been some cause or causes of error unnoticed. I intend, therefore, to repeat the 

 whole experiment, and hope to be able to make a further communication. 



In conclusion, I must say— and I say with exireme gratitude — that if the 

 experiment be in any way satisfactory, it "is chiefly due to the very able and kind 

 instructions given me by Sir Wm, Thomson and his assistants in carrying out the 

 experiment. 



K K 2 



