EuTHERFOED. — Ou Miiguetic Viscosity. 195 



with the period is not yet known. The molecule of iron can 

 swing completely round in less than a hundred-millionth part 

 of a second ; but it is quite probable that the magnetizing 

 force required to produce any given induction is considerably 

 greater for a frequency of 100,000,000 than for a frequency of 

 1,000. For very rapid frequencies the screening effects are 

 so great that only a very thin skin of the iron is magnetized, 

 and the effect of successive oscillations makes the interpreta- 

 tion of the results very dithcult. 



Various Uses of the Time-apparatus. 



Not only was the time-apparatus a very simple means of 

 determining the times of rise of currents in circuits when a 

 steady E.M.F. was applied, but with different connections the 

 duration of secondary induced currents at make and break of 

 the primary could be examined under any conditions required. 

 Very interesting information in regard to the screening effects 

 of solid iron in rapidly-changing fields was deduced, and the 

 subject of the gradual decay of magnetic force in magnetic 

 and non-magnetic conductors, when the magnetizing force was 

 removed, was experimentally verified. The behaviour of the 

 magnetic metals when subjected to rapidly-changing fields is of 

 great practical importance, and the need of very fine lami- 

 nation of the iron for high rates of alternation is clearly shown 

 in all the experiments. 



The principle of the time-apparatus can also be used to 

 determine the velocity of projectiles at various points of their 

 path. If two conductors, actmg as shunts to the battery and 

 galvanometer circuits respectively, be placed in the path of 

 the projectile at a convenient distance apart, the time taken 

 to traverse the distance between the two could be readily 

 determined by observation of the amount of rise of the current 

 during the interval. In a circuit of known inductance and 

 resistance, the observed deflection of the galvanometer would 



be proportional to , ~ j " ^ ; *^iifl. since ^r is a constant for the 



circuit, t could readily be determined, and thus the velocity 

 known. This method is purely electrical, and is capable of 

 great accuracy. The determination of the constants of the 

 circuit is a simple matter, and there are no sources of 'error 

 introduced. 



Time of Kise of Currents in Various Circuits. 



In the experiments on magnetic viscosity the times of rise 

 of currents in circuits containing iron were determined. It 

 was observed that the nature of the curve of rise varied greatly 



