ON THP] MEASUKEMl^NT OF MAC4NETIC HYSTERESIS. 
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with the moveable coil is formed by a second jjiece of the same bronze wire, about 
6 centims. long, the total resistance of these two bronze wires being about 6 ohms. 
The time of vibration of the coil so suspended is about 9'5 seconds. It was found 
best to allow the lower bronze wire to be quite slack. The plane of the suspended 
coil then takes up a definite position due to the control of the bronze wires, and 
is unaffected by slight tiltings of the instrument. If the lower wire is pulled tight, 
and if the centre of gravity of the suspended system does not lie on the line joining 
the points of attachment of the bronze wire, a slight tilt of the instrument causes 
the coil to turn through a considerable angle. 
The dynamometer Avas placed so that the axis of the fixed coils was at right 
angles to the magnetic meridian. The earth’s magnetic force had in consequence no 
action upon the suspended coil in its equilibrium position. By diverting the primary 
current from the fixed coils and alloAving the secondary current to flow through 
the susj)ended coil, we found that the earth’s magnetic force produced no deflexion 
of the suspended coil. Our experiments are therefore free from any error arising 
from the action of the eartli. The motion of the coil Avas observed Avith the aid of a 
lamp and scale in the ordinary ^vay. 
A little care is required in soldering the phosplior-bronze Avire to its attachments. 
If the soldering bit is too hot, and if it conies into contact Avith the bronze, the 
solder alloys AAuth the bronze to such an extent that the latter becomes A^ery Aveak. 
FolloAving a suggestion of Mr. W. G. Bye, aa^o found that a strong joint is easily 
made in the folloAving manner. Tlie stout Avire to Aidiich the fine bronze Avire is to be 
soldered has a fine hole drilled along its axis ; this hole is filled Avith solder which 
is kept melted by applying the soldering bit to the side of the Avire. The fine bronze 
AA'ire is then inserted into the melted soldei', and the soldering liit is at once removed. 
The result is a A^ery satisfactory joint. 
The Reversing Keys. 
§ 28. In the earliest experiments an ordinary (mercury) rocking key Avas used. Its 
defect is that, Avhen contact is broken, the high resistance of the spark causes the 
current to sink to zero A'ery rapidly. When contact is re-established the current 
rises at a rate depending upon the self-induction and the resistance of the circuit. 
§ 29. To avoid all sparking, and to cause a very gradual change of the current, we 
next tried a liquid commutator similar to that used by Professor Saving. A 
drum of insulating material, cai'rying tAvo cojiper plates. A, B, revolves between tAvo 
other plates, C, D, in a vessel containing a solution of copper sulphate. The plates 
C, D are connected to the battery, and the plates A, B to the piimary circuit. The 
current Hows in one direction in the circuit Avhen A is close to C, and in the opposite 
direction Avhen A is close to D. The reversal is thus very gradual. But the key 
was not altogether convenient, and its use Avas abandoned. 
§ 30. In a third arrangement an ordinary (mercury) rocking key Avas used, a non- 
