514 PROF. JACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 
apparatus, are ina 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 
1Linch in diameter by 29 inches in length, weighing 142]bs., 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 14 line thick and 35 feet long. The magnetic power 
was measured by means of asteelyard, 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 sufficiently 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 with 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 per cent. of concentrated 
sulphuric acid of the 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 necessary 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 
