486 



Transactions. — Miscellaneous. 



Ordinates. 

 8-5 

 12-5 

 14 

 15 

 15-3 

 15-6 

 15-5 

 15 

 14 

 12 

 10 

 8 

 6 

 4 

 2 

 1 

 0-5 



The steady deflection at first was 85. As the iron commenced 

 to be eaten away the deflection rapidly rose, and reached its 

 maximum, 156. It remained stationary for a short interval 

 at its maximum value, and then rapidly decreased down to 

 zero. When the deflection had fallen to zero the needle was 

 removed, its diameter measured, and found to be 0'013in. 

 The depth of magnetic penetration was therefore about 

 O0095in. 



Now, from the results of experiments on the eating-away 

 of uniformly-magnetized needles, we see that the depth to 

 which the iron is dissolved is proportional to the time. Since 

 in 200sec. the depth dissolved was - 0095in., the rate of 

 solution = 0-000047in. per second. 



If I represent intensity of magnetization of a thin cir- 

 cular shell, distance r from centre of the needle, and M the 

 deflection of the magnetometer at any instant, — 



Then 



/" 



r is 



Ittt . dr is 

 dM 



: aI toM 



. al 



to 



dr ' 



If a be radius of needle at first, it has been shown that 

 (a — ?•) is : : al to t (the time of action of acid) . 



Let a — r = fc . t, 



then — dr — kdt, 



and, substituting in equation (1), we get — 



I (a - kt) 



is 



. al 



dM . , . 



to -jt- , since dr is 



lis 



.al 



to ■ - 



a — kt 



dt 



dM 



dt 



. al 



to dt 



