THE SMITHSONIAN INSTITUTION. 351 



On closing tlie battery, a few moments elapsed before the needle 

 came to rest, from very rapid oscillation ; and even after the oscilla- 

 tion bad ceased it went back slowly, and was tolerably stationary at 

 26°, which is the first entry in the table. A copper wire was then 

 inserted, of which resistance, by previous experiment, had been found 

 equal to that of 7.2 metres of the normal wire. The needle came to 

 rest at 12°, but after a short time went back to 11.5°. 



The copper wire was then removed from the circuit, when the de- 

 flection was 24°, &c., &c. 



The brass wire, which reduced the deflection to 5°, had a resistance 

 equal to that of 29.2 metres of the normal wire. 



Thus we see that the current of this element, after the first oscilla- 

 tion, remains tolerably constant — at least, so much so that approxi- 

 mately accurate estimates can be made for computing the electro- 

 motive force and resistance to conduction. While, on the one hand, 

 the electro-motive force is considerably weakened by the current, on 

 the other the resistance is not great, even with very weak acid. 

 Where it is not important to make exact measurements, and when a 

 steady current is not required for a long time, the zinc and copper 

 battery may be advantageously applied to many galvanic experiments. 

 If elements with large surfaces are necessary, the form of Hare's 

 spiral is to be preferred. 



The force of the polarization is dependent, most probably, upon the 

 strength of the current, though accurate researches on this subject are 

 yet wanting. 



The reason why batteries with one liquid are not constant is to be 

 sought in the polarization of the negative plate, and this is obviated 

 as much as possible in the so-called constant battery. Yet the 

 strength of the current of the constant battery gradually decreases, 

 by leaving it closed for a long time, because the liquid gradually 

 changes — the dilute sulphuric acid becoming converted, by degrees, 

 into a solution of sulphate of zinc. A corresponding change in the 

 nature of the liquid takes place in all batteries, without exception, 

 and it is only to be avoided by renewing the liquid from time to time. 

 An arrangement might be so made that the heavy solution of sul- 

 phate of zinc would flow off slowly from the lower part of the vessel, 

 and the fresh acid flow in above at the same rate. 



A circumstance which acts quite injuriously in all batteries without 

 porous partitions is, that, in consequence of the current, the sulphate 

 of zinc solution is decomposed, and metallic zinc deposited on the 

 negative plate, whence, during a protracted action of the battery, its 

 electro-motive force must decrease more and more. 



The constancy of the battery current depends essentially upon its 

 strength. Feeble currents, like those obtained by using very dilute 

 acid, and with great resistance included in the circuit, remain constant 

 for some time ; while, by using stronger acid and less resistance, the 

 strength of the current must necessarily decrease far more rapidly. 

 Hence, if it be desired to compare different batteries, with reference 

 to their constancy, equal resistance and like acid must be used. Neg- 

 ect of these conditions may have been the occasion of numerous 

 rrors in regard to the constancy of single batteries. 



