514- PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 



a]iparatus, are in a direct ratio to the active surfaces, it seemed to me of 

 great importance to establish the relation between the surface of a voltaic 

 pair and the weight capable of being supported by a bar of soft iron sub- 

 mitted to the magnetizing power of the current. A bar of soft iron 

 liinch in diameter by 29 inches in length, weighing 14^1bs., was 

 bent into a horseshoe, so that the centres of the branches were seven 

 inches apart. The bar, covered with silk, was covered by a helix of 

 copper wire of 1^ line thick and 35 feet long. The magnetic power 

 was measured by means of a steelyard, and a weight supported by rollers, 

 in order to slip easily over the arm of the lever. The surface of the soft 

 iron armature was of a somewhat convex form, in order that the ex- 

 tremities of the branches, forming a flat and smooth face, should only be 

 touched in the direction of an edge, the position of which formed a right 

 angle with the direction of the lever. The armature was in contact with 

 the extremities of the branches when the lever was placed horizontally. 

 Upon the latter had been marked a scale, the divisions of which indicated 

 the thirtieth part of the sliding weight, to which was affixedan index: it 

 was easy to estimate the tenths of these divisions. I had taken the neces- 

 sary precautions to avoid as much as possible the errors of observation 

 arising from the disposition of the apparatus. I shall not enter here into 

 the details of the construction of this rather complicated apparatus, which 

 I intend to give elsewhere, as it may be useful for experiments of this 

 kind. The electromotors which I employed consisted of copper troughs 

 three quarters of an inch wide, and sufiiciently large to enable me to 

 immerse in them respectively the plates of zinc of 4, 16, 36, 64, 100, 

 144 square inches. The contact of these last Avith the copper was pre- 

 vented by the interposition of pieces of wood. The conducting liquid, 

 of which I had previously prepared a sufficient quantity to serve for a 

 series of experiments, was acidulated with ten percent, of concentrated 

 sulphuric acid of ihe specific gravity of 1-840. The experiments, with 

 the same voltaic pair, were made without interruption ; but after each 

 one precautions were taken to cleanse carefully the zinc plates, to wash 

 the trough with water and to renew the liquid, in order to restore the 

 same state of action. But subsequent observations convinced me that 

 the original state is restored more certainly by exposing the plates, and 

 especially the negative one, to a current of air, until it is perfectly 

 dried. It will then be no longer necessaiy to renew the liquid so fre- 

 quently, especially when the observation is confined to the primitive 

 state. It must be acknowledged that I subsequently found the copper 

 troughs to be ill adapted for electro-magnetic experiments; concentric 

 cylinders, which may be plunged in the liquid, are much better. These 

 cylinders must be fixed firmly enough to remain at the proper distance, 

 without recurring to the interposition of wood or of any other insu- 

 lating matter. Much more constant galvanic effects may be obtained 

 if the space occupied by the liquid between the two metals be not 



