Permanent Magnetic Field, 311 



to test constancy, it was evident that the subsequent tests 

 would he very much like the first, and that by the above 

 method of working I might neglect damping, and also any 

 consideration of the law of the galvanometer deflexion. 



As the observations run through summer and winter they 

 had to be corrected for temperature variation. The coefficients 

 used were those now generally accepted, namely : — For the 

 Latimer-Clark cell O077 per cent, per 1° C ; for the copper 

 wire of the galvanometer 0*38 per cent, per 1°; and for the 

 German-silver resistances 0*044 per cent, per 1°. 



It now remains to give the results obtained. As my 

 present purpose is simply to show how far constancy of field 

 has been obtained, it is not necessary to give many details. 

 Before giving the results it will be proper to state the chief 

 differences between the instruments numbered I., II., and III. 

 in the Table. 



Hemisphere No. I, — Air-gap a little more than ^ inch 

 and not quite uniform. Magnetized by flashing current from 

 four accumulators, July 29, 1890. Heated and cooled several 

 times on different days. Very small decay to August 11, on 

 which day the number of lines = 21,007. Next day, August 

 12, 1890, lines =21,120. Seven months later, March 14, 

 1891, lines = 21,035. Accident to coil caused me to take to 

 pieces and magnetize afresh. A current flashed through at 

 a temperature of about 50° C. 



Hemisphere No. II. — Most highly finished. Air-gap rather 

 less than T X g inch. Magnetized by flashing. Lines about 

 34,000. Nearly 50 per cent, greater than in I. This 

 showed tendency to fall. I therefore adopted the method 

 known as "reducing," by sending reverse current through 

 magnetizing coil. Reduction of field about 5 per cent. 



Hemisplwe No. III. — Magnetic system not so well sup- 

 ported as in two previous instruments. Magnetized by flash- 

 ing current, and then " reduced " about 20 per cent. 



The figures in the column headed " No. of Lines J ' are 

 obtained by the expression 



^_100CVR tf 

 t ' d ; 

 where C = capacity of condenser in microfarads, 

 V = potential difference in volts, 

 R= resistance in circuit with magneto-coil, 

 t= number of turns of wire on magneto-coil, 

 d ! ,d = throws from magneto-inductor and condenser 

 respectively. 



The factor 100 translates from practical to absolute units. 



