12 On Prof. Himstedt's Determination of the Ohm. 



Himstedt has considered this matter. It admits of a very 

 simple test, all that is necessary being to deflect the needle 

 into its oblique position with an external permanent magnet, 

 and then to allow the induced currents to pass, suppressing the 

 interruption of the secondary contact. Both make- and break- 

 induced currents would then pass, whose mean value is 

 zero ; and any deflection of the needle under these conditions 

 would be a sign that its magnetism fluctuated and that the 

 evaluation of either stream alone would be vitiated. 



An interesting feature in Prof. Himstedt's work is the 

 arrangement of the primary and secondary coils, of which the 

 former is a long solenoid embraced by the latter. The fact 

 that as regards the secondary the induction-coefficient depends 

 sensibly upon the number of turns only, without regard to 

 radius, is much in its favour. Any one who has had to do 

 with the measurement of coils will appreciate too the advan- 

 tage of reducing the primary to a single layer. There are, 

 however, disadvantages in this arrangement which must be 

 kept in sight. I will not dilate upon the use of a wooden 

 core on which to wind the primary, though I should think it 

 hardly safe. But assuming that there is no important uncer- 

 tainty as to the value of R (the mean radius of the primary) , 

 though it should be remarked that it occurs in the formula as 

 a square, nor in the data relating to the secondary, we have 

 still to consider the factor K, expressive of the number of 

 turns per unit length in the primary. So far as appears, the 

 value of this quantity is obtained by simply dividing the whole 

 number of turns, 2864, by the measured length, 135*125 cm. 

 Now there is here a tacit assumption either that the wire is 

 wound with perfect uniformity, or that we have to deal only 

 with the mean value. The latter alternative is manifestly in- 

 correct, since the central parts lying nearly in the plane of the 

 secondary are necessarily more effective than the remoter parts. 

 In point of fact the simplicity of this arrangement is more 

 apparent than real, relating rather to calculation than to 

 measurement, as I have already had occasion to remark* in 

 connection with a somewhat similar use of a long solenoid in 

 Mascart's determination of the electrochemical equivalent of 

 silver. How far uniformity was attained in the present case I 

 have no means of judging ; but where the successive turns are 

 merely brought into contact with one another I should not 

 expect a high degree of precision, if only because the thick- 

 nesses of the wire and silk are liable to vary. Again, it may 

 be possible to verify the uniformity a posteriori, or to obtain 



* Phil. Trans. 1884, p. 413. 



