4 JACOBI OX THE APPLICATION OF ELECTRO-MAGNETISM 



avoid them Avhen we are examining closely, by different pro- 

 cesses, subjects in which we are deeply interested. 



27. 

 However these experiments may be conducted, one very im- 

 portant consequence results — namely, that the above-mentioned 

 law of Mr. Barlow must be limited to very weak inductive forces, 

 as is the case with terrestrial magnetism. In employing more 

 energetic electric currents we find a more considerable difference 

 between the magnetic power of tubes of various thickness. For 

 instance, in considering the cylinders B and D, the weights 

 of which are as 2 : 1, we shall find that their magnetic power, 

 under the influence of a weak current proceeding from a pair 

 of 21^ square inches, is as 90 : 89 ; on employing a pair of plates 

 24 inches square, as 90 : 82 ; and with a pair of a 100 inches, 

 as 94 : 7G. Thus, with respect to the i-elation existing between 

 the magnetic power and the masses, there will always be a great 

 advantage in employing hollow tubes for the motive parts of mag- 

 netic machines. This advantage will only be destroyed at that 

 point, where the magnetic forces, divided by the masses, become 

 equal quantities ; although it is still a gi'cat question whether 

 this limit can ever be attained. A separate experiment which I 

 made on the two tubes B and D, and which I am about to mention, 

 appears to support the opinion, that this limit of constant rela- 

 tion is very distant, or that electric forces of considerable energy 

 wovdd be required to approximate to this term. I employed a vol- 

 taic batteiy, consisting of 12 plates of amalgamated zinc^ each of 

 72 square inches, and inclosed in a copper sheath. The pairs were 

 combined into a batteiy, and the apparatus was immersed in a 

 trovigh of baked earth, without partitions, and filled with the mix- 

 ture of sulphuric and niti'ic acids, recommended by Mr. Faraday. 

 The force of this apparatus was very powerful. The helix, form- 

 ing the conjunctive w ire, and wound round the cylinder, became 

 highly heated. Under the influence of such a cuiTent, the tube 

 B, of 2i lines in thickness, was capable of supporting a weight 

 of 95lb., Avhile the cyhnder D, of 1 line in thickness, could only 

 lift 5 lib. In a second experiment the forces were as 94:50, 

 From this it is evident that the advantage is always on the side 

 of the cylinder of least mass. I have purposely avoided bring- 

 ing tlie solid cylinder into this comparison. It will subsequently 



