EuTHERFOED. — Oa Magnetic Viscosity. 201 



In order to experimentally test the rate of decay of induc- 

 tion in such a cylinder, a solenoid lOcni. long was wound 

 uniformly with wire, ten turns to the centimetre. A secondary 

 coil was wound over the primary, sufficient to give a con- 

 venient deflection in the galvanometer. On breaking the 

 steady current flowing through the primary an induced cur- 

 rent circulates through the secondary, and the duration of 

 this secondary current depends on the resistance and induct- 

 ance in the secondary circuit. 



If sufficient non-inductive resistance be added in the 

 secondary the duration of the induced current may be readily 

 reduced to less than 7,-05-573 °^ '^ second. 



If the copper cylinder be now introduced into the solenoid 

 the duration of the secondary is considerably prolonged, and 

 its curve of rise and decay may be determined by the same 

 method which has been used before. 



The arrangement for the experiment is exactly the same 

 as in fig. 4. 



1,000 ohms non-inductive resistance was added in the 

 secondary circuit, and the duration of the secondary was less 

 than ^Q-5— 5 of a second. The solid cojjper rod was now placed 

 in the circuit, and at break the iiiduced current was found to 

 be considerably prolonged, due to the time taken for the 

 magnetic force in the cylinder to decay. 



From the fact that when there is no metal the whole 

 current has passed in less than ^^^^ of a second, we see that 

 the current circulates in the secondary almost instantane- 

 ously after the lines of force pass out of the primary. When 

 the copper cylinder is placed in the solenoid the quantity of 

 electricity that flows in the secondary for any definite interval 

 is proportional to the number of lines of force that have passed 

 out of the primary in that interval. 



Let N — total induction through secondary ; let a and b 

 be the areas of primary coil and copper cylinder respectively: 



The induction through the copper = — . N. 



The part of the induction N (l j decays very suddenly ; 



but the induction through the copper decays gi-adually. 



On pages -356, 357, "Eecent Eesearches," a table is given 

 for the theoretical calculated values of the rate of decay for a 



series of values ^ where 



or 



T = , where r is radius of the cylinder. 



Now, for this experiment, assuming yu, = 1, o- = 1,600, 

 T = 0-0069sec. approximately : — 



