230 MB. S. BIDWELL ON CHANGES PRODUCED BY MAGNETISATION. 



and of magnetising current being nearly proportional when the experiment was stopped 

 with a field of 1400 units. The actual elongation was then about four times as much 

 as the maximum elongation of the iron rod. 



It is clear that so far as cobalt is concerned the subject is not yet exhausted, though 

 the further experimental investigation of it would not be easy. A limit to the inten- 

 sity of the magnetic field, which can be produced by a coil of given dimensions, is 

 imposed by the heating effect of the current,* and this limit was practically reached 

 in my experiments. With a larger coil, and a suitably adapted instrument, stronger 

 fields could undoubtedly be obtained without either serious heating or sacrifice of 

 uniformity ; but the battery power necessary for such a coil would be dispropor- 

 tionately great, so great, indeed, that for any considerable increase of the field it 

 would not be easily obtainable. 



It is unlikely that a field suitable for these experiments could be produced by means 

 of an electro-magnet. For the present, therefore, our knowledge as to the behaviour 

 of cobalt under magnetisation must remain incomplete. 



In conclusion I have to express my hearty thanks to Mr. PREECE for having so 

 freely placed at my disposal the resources of his valuable electrical installation. 



The quantity of heat generated in the coil per unit of time varies as the square of the magnetic field. 



