1879.] 



On an Induction-- Currents Balance. 



63 



20 centims. or more in length, so arranged by a winding key that we 

 can apply a strain to this wire, we find a magnetic conducting value, 

 unstrained, of 100, but on applying a slight strain its value rapidly 

 increases, being more than double at its breaking point. If during 

 this strain we strike the wire, we hear its musical tone, and no matter 

 how much we may wind or unwind it, provided we do not pass its 

 limits of elasticity and similar wire is used, the same musical tone 

 will invariably give the same magnetic value. Thus the note A, or 

 435 complete vibrations per second, gave always the magnetic value of 

 160, or 60 per cent, increase of power over the unstrained wire. If 

 whilst this wire is strained, giving the value 160, we magnetise it by 

 drawing over it a strong compound magnet, the note remains the same, 

 showing no difference of tension, but its magnetic value has fallen 80°, 

 being now 80 instead of 160 ; and this wire can never again be brought 

 by strain up to its previous high conducting powers. Now as we have 

 seen that magnetism produces no-change in hard tempered steel, but that 

 in soft iron it produces a change very analogous to that of temper ; and 

 as the effect of strain would be also to harden the fibres by bringing them 

 all parallel to the line of mechanical strain, and as this improves its con- 

 ducting power, while magnetism instantly destroys all the benefits of 

 the longitudinal mechanical strain, we can only draw the conclusion 

 that magnetism produces a strain analogous to temper but contrary to 

 that of the longitudinal mechanical strain ; in other words, that the 

 magnetic strain is produced perpendicularly to its lines of force. 



This view is sustained by the effects of torsion ; for if, in place of 

 straining the wire, it is twisted, instead of increasing, it rapidly 

 decreases in magnetic conductive value, each turn or twist diminishing 

 its power of conduction in a remarkably constant line of decrease. 

 At 80 turns of this wire there was a decrease of 65 per cent. ; at 85 

 turns the wire broke, and on testing it to see if magnetism had any 

 decreasing effect on it, I found that it produced no change whatever ; 

 but this twisted soft iron wire had now remarkable permanent retain- 

 ing powers of magnetism, being superior to tempered cast steel. 



Again, if we take three similar pieces of soft iron wire, leave the 

 first for comparison in its natural condition, strain the second by a 

 longitudinal strain until it is broken, and twist the third by a torsion- 

 key until it also is broken ; we find on magnetising equally these three 

 wires, and allowing ten minutes' repose, that the first or untouched 

 wire has a retaining power of magnetism of 100, the second only of 

 80, and the third, or twisted wire, of 300. I hope, by the light thus 

 given, soon to be able to produce a magnet whose force shall be greatly 

 in excess of what we have hitherto possessed ; our difficulty at present 

 being that in order to temper steel, we must heat it to redness, and 

 this allows the molecules to rearrange themselves contrary to the 

 object we have in view. 



