306 The Rev. T. R. Robinson's Experimental Researches on the 



think any change of sign occurs: were it otherwise I must have noticed its 

 efiFect ; as in many of these experiments the magnet has been heated by the 

 current above 100°, and an increase oi L must have been produced contrary to 

 all my experience. I may, however, have occasion to re-examine the question, 

 and will not neglect it. 



The fifth column gives L, the number of pounds required to lift the keeper, 

 obtained by reducing the observed number to 60" by the coefficient t. It may 

 seem an easy matter to obtain this, but no one who has not tried it will be pre- 

 pared for the many precautions that are necessary. 



1. The utmost stability in the apparatus, absence of tremors, and dehcacy 

 of touch, are required. With a heavy lift, when approaching the limit of adhe- 

 sion, the agitation caused by a step, the shutting of a distant door, or the action 

 of a gust of wind on the building, will determine a break of contact, with a 

 deficiency of 10 or even 20 lbs. 



2. These magnets (and it is the case also with permanent magnets) will 

 bear a much greater load if the strain be gradually increased, than if it be ap- 

 plied abruptly, the difference being sometimes 40 lbs. Therefore the weight of 

 the steelyard must be slided along very gradually (and I need scarcely say with 

 cautious handling), and allowed to rest at each step a few seconds, as it were, 

 to let the acting forces adjust themselves. I do not see why this should be, 

 unless, perhaps, the state of tension which is produced favours the development 

 of magnetism, but the fact is very striking ; when the keeper is detached and 

 immediately replaced, it will not nearly resist the load, even if that be upheld, 

 and then lowered to its bearing. 



3. Time is an important element : I do not think any current whicli the 

 wire of this magnet can conduct is capable of developing its full power in a few 

 seconds. With the highest power which I have applied it must act for five 

 minutes at least, and from F= 0'3 downwards for full fifteen. This has been 

 noticed, though in a far less degree, by Faraday, who observed the circular po- 

 larization caused by the action of electro-magnets on dense glass to increase 

 for a minute and a half after making the contact. That, however, is not a very 

 delicate test ; and as the poles of his magnet were not connected by a keeper, 

 the molecular excitement must have been far less intense than in this case. 

 As a specimen of this sluggishness of inductivity (which, by the way, is a 



