Measure of the relative Tension of Electric Currents. 269 



a current of electricity, which when measured at the torsion 

 balance was found equal to 20 degrees ; on subjecting this cur- 

 rent to a secondary wire, 7 degrees passed it. Its tension 

 might therefore be represented by 



•3500. 

 A second pair was now added in conformity to the first, 31 

 parts passing ; but when subjected to the secondary wire, 1 8 

 were indicated. The tension had now become 



•5806 : 

 In the same way, by adding three more pairs, the tension 

 rose to -6346. 



It must now be borne in mind, that the numerical deter- 

 minations thus procured are entirely conventional; their abso- 

 lute value depends upon the resistance of the secondary wire, 

 and they therefore only express the relative condition of dif- 

 ferent currents. 



As a considerable advantage will be gained, and much re- 

 petition avoided, by here indicating the mode adopted for 

 procuring the following measures, I shall describe at once 

 some modifications and additions which are necessai-y in the 

 torsion balance, the instrument generally employed. 



The voltameter has of late come much into use in investi- 

 gations of this sort, but when compared with the torsion ba- 

 lance, the latter is much more speedy and certain in its in- 

 dications, and should generally be preferred. In point of fact, 

 the indications of the two instruments are entirely of a dif- 

 ferent character ; the magnetic needle shows the quantity of 

 electricity that is passing in each indivisible portion of time, 

 the voltameter the quantity that has passed at the end of a 

 finite time. In the conditions of the action of the one, time 

 enters as an element, in the other it does not. 



By applying a glass thread to the needle, the late Dr. 

 Ritchie greatly improved the accuracy and general utility of 

 the galvanometer; but even with that addition, unless certain 

 precautions are taken, the instrument will not work satisfac- 

 torily; the motions of the needle are too versatile, and the 

 tremulous state of vibration into which it may be thrown, are 

 insuperable barriers to accuracy of measurement. A cylin- 

 drical trough filled with water is a perfect and admirable re- 

 medy for these difficulties. 



Another difficulty, which is very generally overlooked, is 

 the excentric position into which the thread is liable to be 

 cast, when the upper micrometer has moved. The construc- 

 tion of the instrument requires, that the axis of motion of the 

 upper micrometer, the axis of the glass thread, the axis of the 

 spindle carrying the needles, and the vane, should be in the 



