avoid them when we are examining closely, by different processes, subjects in which we are deeply interested.
27.
However these experiments may be conducted, one very important 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 and , 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 2½ 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 : 76. Thus, with respect to the relation existing between the magnetic power and the masses, there will always be a great advantage in employing hollow tubes for the motive parts of magnetic 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 great question whether this limit can ever be attained. A separate experiment which I made on the two tubes and , and which I am about to mention, appears to support the opinion, that this limit of constant relation is very distant, or that electric forces of considerable energy would be required to approximate to this term. I employed a voltaic battery, consisting of 12 plates of amalgamated zinc, each of 72 square inches, and inclosed in a copper sheath. The pairs were combined into a battery, and the apparatus was immersed in a trough of baked earth, without partitions, and filled with the mixture of sulphuric and nitric acids, recommended by Mr. Faraday. The force of this apparatus was very powerful. The helix, forming the conjunctive wire, and wound round the cylinder, became highly heated. Under the influence of such a current, the tube , of 2½ lines in thickness, was capable of supporting a weight of 95lb., while the cylinder , of 1 line in thickness, could only lift 51lb. 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 bringing the solid cylinder into this comparison. It will subsequently
