Determination of the Ohm. 11 



tioned, in passing, that the object was to obtain, through the 

 galvanometer, a stream of charges of a condenser, separated 

 from the discharges, with a view to the determination of v 

 (the ratio of electrical units) . Dippers in electrical connec- 

 tion with the body of the fork (and by means of a wire attached 

 to the stalk with one pole of the condenser) were carried on 

 both the upper and lower prongs. Underneath these, mercury 

 cups were so arranged that the vib rating-fork was in contact 

 with them alternately, but never with both at the same 

 time. One of the cups was connected with the insulated pole 

 of the battery and the other with the earth. The fork was 

 driven by a current entirely insulated from it. It was found, 

 however, that the contacts could not be made perfect, and the 

 direct use of the fork was abandoned in favour of a commu- 

 tator with platinum contacts driven by the fork*. This form 

 of the apparatus is unsuitable as a separator of induced 

 currents ; and I was inclined to favour the observation of a 

 single induced current with a ballistic galvanometer as carried 

 out by Rowland, and afterwards by Glazebrook. 



It is to be presumed that the contact difficulty has been 

 overcome by Prof. Himstedt; and my principal reason for 

 mentioning it is that I found it particularly capricious and 

 insidious. The galvanometer indication would often remain 

 steady for minutes together, and then suddenly change. It 

 would be interesting to know whether Prof. Himstedt has 

 met with any behaviour of this sort. 



The next question that I wish to raise relates to the 

 measurement by the galvanometer of a series of induced 

 currents, each of short duration. On page 421 there is a 

 reference to " cross magnetization "" that I do not quite under- 

 stand. I have myself | objected to the use of a ballistic gal- 

 vanometer, on the ground of the tacit assumption that the 

 needle at the moment of the impulse, when subject to a power- 

 ful cross-magnetizing force, retains its axial magnetization 

 unaltered ; but in the method of Prof. Himstedt the question 

 assumes a different shape. In this case the needle stands in 

 an oblique position, and we have to consider whether the axial 

 magnetization does not alter under the action of a force 

 having a sensible axial component%. In all probability Prof. 



* " On the Determination of the Number of Electrostatic Units in the 

 Electromagnetic Unit of Electricity," J. J. Thomson, Phil. Trans. 1883, 

 p. 719. 



t Phil. Trans. 1882, p. 670. 



% " On a Permanent Deflection of the Galvanometer Needle, &c," B.A. 

 Report 1868. Phil. Mag. Jan. 1877. Chrystall, Phil. Mag. Dec. 1876. 



