1835] WRITINGS OF JOSEPH HENRY. 85 



at the same time several large magnetic needles on the dif- 

 ferent parts of the apparatus. These instantly change their 

 direction when the second pair of decomposing plates touch 

 the solution. 



At first sight it might be supposed that there would be 

 some difficulty in entering the several plates into their re- 

 spective cells, but this is obviated by the precise movement 

 of the platform on which the troughs stand. Its horizontal 

 position is adjusted by four screws (c c fig. 1), and its corners 

 slide in grooves in the upright posts of the large frame. Be- 

 sides this, when the plates are once entered, they are not re- 

 quired to be entirely withdrawn from the cells until the end 

 of the serfes of experiments ; since the acid descends as the 

 plates are withdrawn, and finally fills but little more than 

 three-fourths of the capacity of the cells. When a plate ac- 

 cidentally catches on the side of the cell, the battery to 

 which it belongs is gently raised in its place and the plate 

 adjusted. 



This apparatus readily furnishes the means of making 

 comparative experiments on the difference produced by par- 

 tial and perfect insulation. When no higher degree of 

 intensity is required than that afforded by eight pairs of 

 plates, perfect insulation is obtained by the eight separate 

 troughs. In higher degrees of intensity the partitions in the 

 troughs furnish the means of perfectly insulating forty -eight 

 of the elements : this is effected by simply charging with 

 acid every other cell in each of the troughs, and connecting 

 the corresponding element by conductors, which pass over 

 the intermediate elements without touching them : with this 

 arrangement we have six cells in each trough separated from 

 one another by a cell without acid, or in effect by a stratum 

 of air. For comparison with these a set of troughs has been 

 constructed without partitions. 



The want of perfect insulation is not very perceptible in the 

 common experimentsof the deflagration of large and perfect 

 conductors ; but where the decomposition of a liquid is at- 

 tempted, or the battery required to act on a small or imper- 

 fect conductor, the loss of power is very great, the apparatus 



