TRANSACTIONS OF SECTION A. 



281 



liquid, a series of currents are flowing in one direction below it, and a correspond- 

 ing deflection takes place ; when, however, it is lowered deeper into the solution 

 a certain number of currents are flowing below it tending to deflect it in one 

 direction, and a certain number are flowing above it tending to deflect it in the 

 opposite direction, and its permanent deflection is due to the electro-magnetic 

 effect of the difference between the two. When these become equal, as they are 

 when the needle is at the middle of its depth, their effects on the needle are 

 balanced and neutralised, and no deflection takes place ; and when that point is 

 passed the currents above the needle are in excess of those below it, and a corre- 

 sponding deflection in an opposite direction is given to the needle. 



Professor Hughes, by placing in the circuit of a battery an apparatus, such as 

 a clock-microphone, or a key, by which an intermittent or undulatory character 



may be given to its current, and holding one side of a rectangular coil of wire in 

 circuit with a Bell telephone over one of the cells of his three-cell battery, a 

 secondary intermittent or undulatory current was indxiced in the coil by that 

 portion of the primary circuit transmitted through the cell, and a corresponding 

 ticking was heard in the telephone. 



In both these experiments, however, the effects observed must be attributed 

 rather to the external current of the other cells than to the internal current of the 

 cell under examination ; and the author is unaware that any successful attempt has 

 hitherto been made to construct an instrument which shall utilise the ivhole of the 

 internal current of a single voltaic cell for the production of electro-magnetic 

 effects. "While engaged in some experiments a few years ago it occurred to the 

 author that if a voltaic cell were divided into two portions, having the zinc element 

 in one portion, and the positive element in the other, and the solution contained in 

 the one portion were connected to that in the other by a tube filled with the same 

 liquid, the tube being coiled round a magnetic needle, a deflection of the latter, 

 due to the current within the cell being forced by the convolutions of the tube to 

 circulate around the needle, would be produced when the two elements were con- 

 nected together. An apparatus (which was before the section) was then constructed. 

 This instrument consists of two glass test tubes united together by a small tube about 

 two feet long, and convoluted into two circular coils after the manner of a Thomson's 

 Reflecting Galvanometer. Within the coils is suspended an astatic system of 

 magnetic needles, of which the upper carries a light mirror by which its deflections 



