168 



Prof. D. E. Hughes. 



[Dec. 20, 



through a small degree with infinitely less force than would be 

 required to rotate it permanently on either side. This property is so 

 marked and general that we can observe it without any special iron 

 or apparatus." 



In order to observe this in electro-magnets, we must employ an 

 extremely feeble current, such as from one Daniell cell, with an 

 exterior resistance of from 10 to 1,000 ohms, and we then find the 

 folio wing laws hold with every variety of iron and steel : — 



1st. That its magnetic capacity is directly as its softness or mole- 

 cular freedom. 



2nd. That its resistance to a feeble external magnetising force is 

 directly as its hardness, or its molecular rigidity. 



This has proved to be the case upon sixty varieties of iron and 

 steel furnished me direct from the manufacturers, and it was remarked 

 that each variety of iron or steel has a certain point, beyond which 

 annealing cannot soften, nor temper harden ; consequently, if all 

 varieties were equally and perfectly annealed, each variety would have 

 its own magnetic capacity ; or, its specific degree of value when 

 perfectly annealed or tempered, by means of which, we could at once 

 determine its place and quality. 



If in place of several varieties we take a single specimen, say hard- 

 drawn Swedish iron wire, and note its magnetic capacity, we find that 

 its value rises rapidly with each partial annealing, until an ultimate 

 softness is obtained ; being the limit of its molecular freedom. We 

 are thus enabled to study the best methods of annealing, and to find 

 at once the degree of softness in an unknown specimen. A similar 

 effect occurs in observations upon tempering ; from the softest to the 

 hardest temper, until we arrive at its ultimate molecular rigidity. 



We have thus in each piece of iron or steel a limit of softness and 

 hardness. In soft Swedish iron tempering hardens but 25 per cent., 

 whilst mechanical compression (such as hammering) hardens it 50 per 

 cent. In cast steel tempering hardens it 400 per cent., whilst 

 mechanical compression gives but 50 per cent. Between cast steel and 

 Swedish iron we find a long series of mild steel and hard iron, 

 varying in their proportionate degree between the two extremes 

 mentioned. 



In order to carry out these researches, I constructed an instrument 

 which I have called the magnetic balance. It consists of a delicate 

 silk fibre suspended magnetic needle, 5 centims. in length, its pointer 

 resting near an index having a single fine black line or mark for its 

 zero, the movement of the needle on either side of zero being limited 

 to 5 millims. by means of two ivory stops, or projections. When the 

 north end of the needle and its index zero are north, the needle rests 

 at its index zero, but the slightest external influence, such as a piece 

 of iron 1 millim. in diameter 10 centims. distant, deflects the needle 



