74 Mr. R. H. M. Bosanquet on the Magnetic 
crown, and I shall speak of it as crown iron. One ring of 
best Lowmoor has also been examined. 
The steel ring J was forged from the remainder of the bar 
of cast steel from which the divided magnet was made, which 
is called A in my paper I. on Permanent Magnets (Phil. Mag. 
August 1884, p. 142). It was first examined soft, and then 
after hardening. 
The method employed was substantially that of Rowland. 
The rings were uniformly wound with coils through which 
the magnetizing current was transmitted. The current em- 
ployed was a small derived current from the circuit of the 
dynamo machine; so that it could be opened, closed, or 
reversed without upsetting the main circuit. The current 
was measured by means of two galvanometers of Helmholtz’s 
pattern, the one having two coils, the other eighteen. The 
galvanometers were always erected on the brick stand which 
forms the standard position for which values of H are deter- 
mined in the laboratory. 
The observations were all made by reversal. 
A number of induction-coils, varying from 250 to 1, were 
wound round the ring to be examined. In circuit with these 
coils were the ballistic galvanometer and the earth induction- 
coil, for the reduction of the readings to absolute measure. 
Two earth induction-coils were employed. ‘The first had 
21 turns, with a mean diameter of 51°30 centim.; the other 
250 turns, with a mean circumference of 166°53 centim. With 
both of these coils the impulse of the galvanometer due to 
half a turn about a vertical axis in the standard position* was 
about 7°. But in the case of the larger coil, a deflection cor- 
responds to about twelve times as great an induction as with 
the smaller coil, so that a very large range is secured. The 
galvanometer has a circular scale which reads, by reflection, 
directly to 5’ and by estimation to single minutes. 
The mode of calculation adopted is precisely equivalent to 
that of Rowland. Rowland’s Mis current-turns per unit length, 
= a in our notation. Then his expression connecting p, 
the permeability, with the total number of lines of force is, 
lines of force =r" a(n?) : 
and for the magnetic induction, or lines of force per unit area, 
* The positions of rest between which the half turn is made are such 
that the plane of the coil is at right angles to the magnetic meridian. 
Hence the coil cuts the lines of force of H twice in the half turn. 
