62 



Prof. D. E. Hughes. 



[May 15, 



the absorption of work done in inducing the circular currents. This 

 can be perfectly balanced by placing a small coin or disk of silver or 

 copper in opposite coils ; but if an iron wire or rod is placed perpendi- 

 cular to the coils, then increase of inductive force takes place in those 

 coils by the conduction of induced magnetism from primary to secon- 

 dary, and the iron can no longer be balanced by silver, copper, or any 

 non-magnetic metal. The coils must be either removed farther apart, 

 so as to reduce the increased force, or balanced by an equivalent 

 amount of iron or magnetic conduction in opposite coils. 



An interesting case of both reduction and increase of force in the 

 same pair of coils occurs if we place a disk of iron, not in the centre 

 of coils, but in the vacant space between the coils.. We thus reduce 

 the force by 150°. If, in addition to this, we place iron wires per- 

 pendicular and in the centre, there is increase of force, and if this 

 increase is so. proportioned as to be 150°, we immediately restore 

 the balance, and we have here in the same coil two separate pieces 

 of iron, each disturbing the balance and giving out loud tones, but 

 producing no effect whatever, when both are introduced at the same 

 time, complete silence being the result. 



VII. These coils prove what has already been long known, viz., that 

 hard steel has. a far less conducting power for magnetism than soft 

 iron, although the hard steel has a far higher retaining power. This 

 instrument demonstrates a point, which I have not yet seen remarked, 

 that magnetism does not in itself change the conducting power, but 

 that it produces a molecular change of structure in iron, analogous to 

 that of tempering ; for if we balance two soft iron rods against each 

 other, the balance being made perfect by the addition of fine iron 

 wires on the weakest side, we find that on strongly magnetising this 

 bar, by drawing it across a strong compound magnet, and on replacing 

 it in its coil, it has lost 30 per cent, of its conducting power ; or if, 

 instead of magnetising, we make this iron red hot and plunge it in cold 

 water, the loss of conducting power will be very similar — 25 to 30°. 

 If these experiments are repeated upon various degrees of iron ap- 

 proaching steel in character, we find that as it already possesses hard- 

 ness or temper, it is less and less affected by magnetism, until we 

 arrive at hard cast steel, where magnetism no longer produces any 

 change in its conducting power. Erom this I draw the conclusion 

 that the effect of magnetism is very similar to that of temper, and 

 shall show, under the effects of strain and torsion, that magnetism pro- 

 duces this temper or strain perpendicular to the lines of magnetic 

 force. 



VIII. The instrument shows that a remarkable change takes place 

 in the magnetic conducting power of iron and steel on subjecting the 

 wire under examination to a longitudinal strain ; for if we pass an iron 

 wire through the centre of both coils, half a millimetre diameter and 



