MAGNETIC PEEMEABILITY OF NICKEL AND COBALT 59 



There is a difficulty in obtaining a good series of experiments on 

 nickel and cobalt which does not exist in the case of iron. It is prin- 

 cipally Giving to the great change in magnetic permeability of these 

 substances by heat, and also to their small permeability. To obtain 

 sufficient magnetizing-force to trace out the curve of permeability to a 

 reasonable distance, we require at least two layers of wire on the rings, 

 and have to send through that wire a very strong current. In this way 

 great heat is developed; and on account of there being two layers of 

 wire it cannot escape; and the ring being thus heated, its permeability 

 is changed. So much is this the case, that when the rings are in the 

 air, and the strongest current circulating, the silk is soon burned off the 

 wire; and to obviate this I have in these experiments always immersed 

 the rings in some non-conducting liquid, such as alcohol for low tem- 

 peratures and melted paraffin for high temperatures, the rings being 

 suspended midway in the liquid to allow free circulation. But I have 

 now reason to suspect the efficacy of this arrangement, especially in the 

 case of the paraffin. The experiments described in this paper were 

 made at such odd times as I could command, and the first ones were not 

 thoroughly discussed until the series was almost completed; hence 1 

 have not been so careful to guard against this error as I shall be in the 

 future. This can be done in the following manner namely, by letting 

 the current pass through the ring for only a shirt time. But there is a 

 difficulty in this method, because if the current is stopped the battery 

 will recruit, and the moment it is joined to the ring a large and rapidly 

 decreasing current will pass which it is impossible to measure accu- 

 rately. I have, however, devised the following method, which I will 

 apply in future experiments. It is to introduce into the circuit between 

 the tangent-galvanometer and the ring a current-changer, by which the 

 current can be switched off from the ring into another wire of the same 

 resistance, so that the current from the battery shall always be con- 

 stant. Just before making an observation the current is turned back 

 into the ring, a reading is taken of the tangent-galvanometer by an 

 assistant, and immediately afterward the current is reversed and the 

 reading taken for the induced current; the tangent-galvanometer is 

 then again read with the needle on the other side of the zero-point. 

 The pressure of outside duties at present precludes me from putting this 

 in practice. But the results which I have obtained, though probably 

 influenced in the higher magnetizing-forces by this heating, are still 

 so novel that they must possess value notwithstanding this defect; for 

 they contain the only experiments yet made on the permeability of 



