PHYSICS. G13 



gatious that this influence is non-existent, at least so far as the media 

 studied by them are concerned. Hence they consider that Ampere's 

 coefficient lc' in his electrodynamic formulae is probably independent of 

 the surrounding medium. Borgmann has called attention to the fact 

 that he had investigated this question six or seven years ago, using 

 Poggendorff s compensation method to determine the induced electro- 

 motive force. He concluded that the dielectric medium had no influence, 

 but that the magnetic medium had an appreciable influence, the electro- 

 motive force of induction being proportional to the coefficient of mag- 

 netic permeability. Hence Ampere's coefficients depend on the same 

 quantity. (J. Phys., June, December, 1883, II, n, 245, 551.) 



Dorn, using a modification of Weber's second method, the same that 

 Kohlrausch employed, has obtained for the value in absolute meas- 

 ure of one Siemens unit 0.9482 xlO 10 millimeter-seconds. (Wied. Ann., 

 xvii, 773; Am. J. ScL, February, 1883, III, xxv, 148.) 



Pending the decision of the International Electric Congress, called 

 to meet in Paris in April, 1884, many methods have been suggested for 

 the determination of the value of the ohm. Carey Foster has called 

 the attention of the London Physical Society to the results, recently 

 obtained, of a method suggested by him in 1874. The method consists 

 in balancing the electromotive force set up in a coil spinning in the 

 earth's magnetic field, by means of an opposing electromotive force 

 from a given battery. The two opposing circuits through the same 

 wire, R, are composed, the first of the spinning coil and a zero galvano- 

 scope and the second of a battery and an absolute galvanometer, these 

 two circuits meeting at the end of the wire R. In two preliminary 

 trials the values 1.003 and 0.999 were obtained, warranting further ex- 

 periments. Glazebrook has obtained the value 0.9866 for the ohm, or 

 the exact mean of Lord Rayleigh's results, 0.9893, 0.9865, and 0.9868. 

 Lippmann has suggested an electrodynamic method, resembling that 

 of Lorenz. A coil is spun inside a long coil, through which a known 

 current is passing. The resistance to be determined is placed in the 

 circuit of the latter coil. The electromotive force produced by the 

 rotation of the inner coil is balanced along the given resistance by the 

 electromotive force in the outer coil. Gray has proposed a method 

 nearly the converse of that of Weber. It is to hang a coil, the constants 

 of which are known, in a sufficiently intense and uniform magnetic field 

 and find the decrement of the oscillatory motion produced by the induc- 

 tion. Roiti has described a method analogous to that employed by 

 Rowland in 1878. He uses a closed solenoid, in which a primary cur- 

 rent flows, and a galvanoscope so arranged that into it can be thrown 

 at will either a branch of the primary current or induced currents re- 

 sulting from a certain number of interruptions of the^principal circuit. 

 The resistances are so chosen that the deflection of the galvanoscope 

 is the same in both cases. Frolich has discussed the question whether 

 electrodynamic actions alone can serve to measure the ohm. Two cir- 



