MR. J. S. TOWNSEND ON MAGNETIZATION OF LIQUIDS. 
539 
The second part of c 2 was similar in principle, and by means of it smaller changes of 
induction could be made. It had only 1 turn in the secondary, the 6 ends of 1, 3, 
and 6 turns on the outer ring terminating in 6 mercury cups, beside those belonging 
to the larger inductance. The radius of the inner circle of wire was 12'4, and that of 
the outer circles 25 centims. 
The planes of c i and c 2 were perpendicular to one another, each going through the 
centre of the other, so that when the number of coils of in series with the primary 
was altered there should be no change in the magnetic force perpendicular to the 
plane of c 2 , and vice versa. The induction of one circle of the primary on the corre¬ 
sponding secondary is, for 
c v 707, and for c 2 , 65.'“ 
Foucault Currents. 
Objections have often been made to the induction method of finding h by using a 
commutator, as it was supposed that, in the case of conductors, the induced currents 
had an appreciable effect on the current through the secondary circuit. 
This, however, is not the case when a galvanometer is used in the secondary 
circuit, but would introduce an error if a telephone were substituted. 
In order to prove this, let us consider the case of currents generated in a circular 
cylinder by the current i — ( 1 — e — n 4 j starting in the primary, E, It, and L being 
the E.M.F. resistance and self induction of the primary. 
We see from symmetry that the induced currents are in circles round the axis, 
and, if r is the perpendicular distance of any point from the axis, and u the strength 
of the current per unit section, 
u — f (rt). 
Considering the circuit of radius r, and thickness dr, extending through unit 
length of the cylinder, the current flowing round the circuit is u dr, the resistance of 
the circuit is 27rcrr/dr, where <r is the specific resistance of the liquid. 
Therefore 
dN 
Zururu = — — > 
at 
where N is the total number of lines of induction going through the circuit. 
N = 47t 2 | r 2 u dr -j- 47tV £ j udr - j- 47r 2 ?' 2 “ (1 — e L * j. 
Hence we have 
* Maxwell, ‘Electricity and Magnetism,’ vol. 2, chap, iv., App. 
3 Z 2 
