354 On the Applicalion of Magnetism 



cision. — The other mode, that of measuring tlie quantity of 

 water decomposed by a jiair of small galvanic plates, is im- 

 practicable. — The recent discoveries of Professor CErsted have 

 enabled me to construct two instruments, one for discovering, 

 the other for measuring, galvanic electricity, with a delicacy 

 and precision that seem scarcely to admit of limitation. — The 

 construction of the former instrument I have mentioned in a 

 communication I liad the honour to make to this Society some 

 time since ; — I shall now describe a few of the experiments I 

 have as yet been enabled to make with it. — A wire of zinc and 

 another of platina, each — inch diameter, were coated with 

 sealing- w^ax, so as to have merely their extremities exposed : 

 on immersing them in a dilute acid, the circuit being at the 

 same tune completed through the galvanoscope, the needle de- 

 viated so decidedly, as to leave no doubt that a visible eflect 

 would have been produced by wires of less than half the di- 

 mensions of those I employed. — As the compass, though small, 

 was by no means delicate, we may, I think, conclude tiom tliis 

 experiment, that the electricity developed by two metallic sur- 

 faces, each jijj of a square inch, may be detected, and their 

 relations to each other ascertained, by this instrument. 



The minute surfaces, and consequently small quantities of 

 exciting fluid required for experiments with this instrument, 

 offer the means of examining galvanic efiects that have hitherto 

 been unnoticed. 



Of the acids whose galvanic effects I believe have not been 

 examined, I have found, with small disks of zinc and copper, 

 that the oxalic and hydriodic have considerable power; the 

 phosphoric and acetic much less. — The action of strong sul*- 

 phm'ic acid was inconsiderable, the needle being scarcely af- 

 fected ; but on adding a drop of water it deviated through 

 more than half a right angle. — Were the galvanic action owing 

 solely to the electricity developed by the metallic contact, the 

 fluid acting merely as a conductor, according to a generally 

 received hj'pothesis, the effect should be greatest when the 

 stronger acid is used, concentrated sulphuric acid being a far 

 better conductor than when diluted : on the other hand, since 

 zinc or iron are readily oxidated by the action of dilute acid, 

 though with difficulty when it is concentrated, this experiment 

 seems to prove, that the galvanic action depends, not on the 

 conducting, but on the oxidating power of the interposed fluid. 

 — Hitherto I have not had the leisure to form that complete 

 series of the electric relations of the metals towards each other, 

 which this instrument affords the means of doing; yet the ef- 

 fects 1 have observed in two instances, are, I think, so remark- 

 able, that I ought now to mention them. — On using two disks, 



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