THERMO-MAGNETISM AND PYRO-MA ONETISM.

431

In tlie steam-exhaust pipe h is a valve v, to which is connectedthe lever i, by the movement of which tlie valve is openedor closed. In such a case as tins the heat of the fire may heutilized for other purposes after as much of it as may he neededhas been applied to heating the core b. There are special ad-vantages in the employment of a cooling device, in that themetal of the core n is not so quickly oxidized. Moreover, thedifference between the temperature of the applied heat and ofthe steam, air, or whatever gas or fluid be applied as the coolingmedium, may be increased or decreased at will, whereby therapidity of the magnetic changes or fluctuations may be regulated.