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XLVIII. On the Determination of Coefficients of Mutual 

 Induction by means of the Ballistic Galvanometer and 

 Earth- Inductor. By R. H. M. Bosanquet*. 



MY attention was drawn to this subject by tbe paper 

 recently read before the Society by Prof. Foster f. I 

 observed at once that the appliances which I am in the habit 

 of using afford a very simple solution of the problem. 



The ballistic galvanometer has a resistance less than 2 B.A. 

 units, and about 500 turns. I have three earth induction- 

 coils, all having a mean diameter of about half a metre, and 

 the following constants. The resistances are approximate, as 

 they vary so much with temperature. The wires are all 

 cotton-covered copper. 



Number of Total effective Mean circum _ Approximate w 

 turns. area. £ resistance, -dtxt r< 



N. logNA. ference ' B.A. RW ' G - 



42 4-97132 167-325 -6 16 



250 5-74200 166-583 10-8 20 



1000 6-36182 170-005 42 20 



The circular channels are turned in wooden rings framed 

 together in many pieces. They were turned in the Royal 

 Society's lathe in my laboratory. They are mounted so as to 

 turn on vertical axes through half a revolution, iu doing which 

 they are reversed with respect to the horizontal component 

 of the earth's magnetism, and so experience an electrical 

 impulse equal to 2 NAH. 



If the resistance of the circuit be R, a transfer of electricity 

 then takes place, such that 



_2NAH 

 Vo- R 



The various windings of the coils were measured with great 

 care during the construction. The areas were calculated for 

 each layer separately; and the above values of logNA are 

 probably certain to the fourth place of decimals. 



There are two tangent-galvanometers, both on Helmholtz's 

 pattern, having mean circumferences of exactly 1 metre. The 

 one has 2 coils, the other 18. The currents in them are 

 measured by the formula 



C = GHtan0; 



* Communicated by the Physical Society : read February 26, 1887, 

 t Phil. Mag. February 1887, p. 121. 



