1882.] Experiments on the Value of the Ohm. 



87 



about 780 turns, were placed with their mean planes parallel and at a 

 known distance apart. The coefficient of mutual induction between 

 the two can be found from the geometrical data ; let this be M. Let 

 one of the coils be connected in circuit with a ballistic galvanometer, 

 and let R be the resistance in centimetres per second of the circuit. 

 Let a steady current of intensity i be circulating in the other coil — the 

 primary. On reversing this current an induction current, of which 

 2M? . 



the amount is — -, is produced in the secondary circuit, and the 

 R 



galvanometer needle is disturbed from rest ; if (3 be the first throw of 

 the needle, T the time of a complete vibration, X the coefficient of 

 damping, t that of torsion, Gr the galvanometer constant, and H the 

 horizontal intensity of the earth's magnetism, we have 



The galvanometer was then connected in series with a large re- 

 sistance coil, in our case of about 3,000 ohms ; let S be the resistance 

 of the galvanometer and this coil. The two extremities of the re- 

 sistance S were connected with two points in the primary circuit, the 

 resistance between which was about 1 ohm ; let this resistance be Y. 



Then of the primary current i, an amount is transmitted 



through the galvanometer, and if cc be the deflection of the needle, we 

 have 



Eliminating i, G and H, we obtain 



R _ 2-M S + Y tana 



~K 1+ !)( 1+ s) T 



R 



And if R be the value of R in ohms, the ratio — gives us the value 



R 



of the ohm in centimetres per second. 



The coils and galvanometer were wound for this purpose with great 

 care by Professor Chrystal under the supervision of the late Professor 

 Clerk Maxwell. Professor Chrystal's removal from Cambridge pre- 

 vented the completion of the experiments by him. 



For a detailed account of the precautions necessary, the methods of 

 making the observations, and the comparison of the resistance coils 

 used, reference must be made to the paper. 



Each experiment involves eight observations of throw due to the 

 induction current and two of deflection ; the values of the deflection 



