PRESIDENTIAL ADDRESS — SECTION A. 33 



energy stored up in every cubic centimetre of the medium in 

 consequence is numerically equal to — —- BH. 



OTT 



From this it can easily be shown, as Ewing has done, that 

 the area of the curve SCS'C'S divided by 47r represents the 

 energy expended in every cubic centimetre of the iron by the 

 current in performing the cycle of operations represented by the 

 curve. This is energy wasted, dissipated in heating the iron 

 during the process of reversal, and it is obvious how necessary 

 it is to keep the amount so wasted as small as possible in the 

 case of armatures of dynamos and cores of transformers. For 

 very soft iron this waste is small, but with hard steel it may 

 be as much as 20 per cent. Professor Ewing has also shown 

 that the amount of energy so wasted by hysteresis can be very 

 much lessened by mechanical vibration. 



The effect of temperature on the magnetic properties of iron 

 is very remarkable. If we insulate the wires round the ring of 

 soft iron with asbestos, and make a series of experiments by 

 the first method, always using the same current in the primary, 

 but gradually increasing the temperature of the iron, we shall 

 find, if the magnetizing current be small, that the permeability, 



/x, of the iron — that is, the ratio — -. — increases slowly with the 



temperature at first. When the temperature is about 600° C. 

 fi begins to increase at an enormous rate, and at 770° is as 

 much as 11,000. The addition of a few more degrees of tem- 

 perature at this point reduces iron to a non-magnetic metal. 

 The relation between the permeability and the temperature 

 for a small magnetizing current is shown on Fig. IV. (PI. II.), 

 taken from Hopkinson. 



The variation of the permeability of the iron would have 

 followed a very different order had we used a large magnetizing 

 force instead of a small one. In this case, instead of increas- 

 ing with the temperature, fx steadily decreases, slowly at first, 

 but when the temperature is about 600° C. ^ decreases more 

 rapidly, and finally at the same temperature as for small 

 currents the iron becomes non-magnetic. (Fig. V., PI. II.). 



In general terms, we may state that for small magnetizing 

 forces increase of temperature makes iron more magnetic until 

 a certain critical temperature, 770° C, is reached, when it sud- 

 dently becomes non-magnetic. 



For large magnetizing forces increase of temperature makes 

 iron less magnetic, its magnetic properties vanishing at the 

 same critical temperature as for small forces. Evidently some 

 very striking change must occur in the molecular condition of 

 the iron at this critical point. It is the temperature of the re- 

 calescencc phenomenon of Barrett, which is interesting enough 

 3 



