ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
5.'I 
at the same time to keep the resistance of the circuit unchanged. Since both coils 
were of copper, their resistances were equal when their temperatures were equal. 
The Magnetising Solenoid. 
§ 24. In our experiments the specimens of iron have taken the form of straight 
wires, and in consequence the magnetising coil has been a straight solenoid. We used 
a straight wire instead of a ring in order to he able to apply tension or torsion to the 
specimen. The solenoid is formed of several independent layers of wire wound upon 
an ebonite tube 47 centims. in length. It is essential that tlie tube shonld be of 
non-conducting material, for otherwise the currents induced in it would cause the 
magnetic force to differ considerably from that calculated from the formula II = IttNCI ; 
this effect would cause a considerable error in the measurement of the hysteresis. By 
combining the independent layers in different ways we could vary the magnetic force 
due to unit C.G.S. current by steps of about 50 from 50'55 to 718'6 C.G.S. nnits. 
The magnetic forces due to unit current in each of the four coils are as follows :— 
AA, oO'oo, BB, 50‘55, XY, 212'2, MN, 405‘1. 
The resistances of the coils are ‘54, ’54, 2'00, and 4'85 ohms respectively. 
The Secondary Coils. 
§ 25. The secondary coils were formed of fine insulated copper wire wound on 
narrow tubes of ebonite or glass, through which the specimen passed. Insulating 
material was used for tlie bobbins to avoid the induction of currents in them. In our 
eailiest experiments it was found that metal tubes caused very large errors by the 
action of the currents induced in them. Several coils were used with the dynamometer, 
the number of windings varying from 300 to 1285; the coil of 1285 turns had a 
bobbin 11 *5 centims. long, thus conforming to the recommendation of ^ 11. The 
mean area of this coil was '785 sq. centim. 
The Badlistic Gcdvanometer. 
§ 2G. The ballistic galvanometer was made by one of ns, with some assistance from 
Mr. W. G. PvE. The magnet system consists of two vertical magnetised wires, each 
about 11‘3 centims. long and 1 milliin. in diameter. These are fixed parallel to each 
other at a distance of 1 centim., the north pole of one magnet l)eing opposite the 
south pole of the other. When parallelism is secured, the magnetic system is 
necessarily astatic, however much the magnets may vary in strength. The system 
is suspended from a torsion head by a single phosphor-bronze wire xo~o'o ’^^ch in 
diameter and 12 centims. in lengtli. The torsion of this bronze wire supplies the 
restoring coiqile. There are four coils, each containing about 250 turns of No. 18 
B.W.G. cojiper wire, the total resistance of the coils in series being 2'14 oliins. Tlie 
coils are arranged so that the upper end of the magnet system is at the centre of the 
