2 Sir H. Davi/ on the Magnetic Pkanometia [Ja n . 



quantity of electricity ; and in both cases they were neither 

 attractive of each other, nor of iron filings, and not affected by 

 the magnet ; and the only proof of their being magnetic, was 

 their occasioning a certain small deviation of the magnetized 

 needle. 



Thus, a large piece of charcoal placed in the circuit of a very 

 powerful battery, being a very bad conductor compared with the 

 metals, would not affect the compass needle at all, unless it had 

 a very large contact with the metallic part of the circuit ; and if 

 a small wire was made to touch it in the circuit only in a few 

 points, that wire did not gain the power of attracting iron 

 filings ; though, when it was made to touch a surface of platinum 

 foil coiled round the end of the charcoal, a slight effect of this 

 kind was produced. And in a similar manner fused hydrate of 



Eotassa, one of the best of the imperfect conductors, could never 

 e made to exert any attractive force on iron filings, nor could 

 the smallest filaments of cotton moistened by solution of 

 hydrate of potassa, placed in the circuit, be made to move by 

 the magnet ; nor did steel needles floating on cork on an elec- 

 trized solution of this kind, placed in the voltaic circuit, gain any 

 polarity ; and the only proof of the magnetic powers of electri- 

 city passing through such a fluid, was afforded by its effect upon 

 the magnetized needle, when the metallic surfaces, plunged in 

 the fluid, were of considerable extent. That the mobility of the 

 parts of fluids ilid not interfere with their magnetic powers as 

 developed by electricity, I proved, by electrifying mercury, and 

 Newton's metal fused, in small tubes. These tubes, placed in a 

 proper voltaic circuit, attracted iron filings, and gave magnetic 

 powers to needles; nor did any agitation of the mercury or metal 

 vrithin, eithei' in consequence &( -mechanical motion or heat, 

 alter or suspend their polarity. 



in. Imperfect conducting fluids do not give polarity to steel 

 when electricity is passed through them ; but electricity passed 

 through air produces this effect. Reasoning on this phitnome- 

 non, and on the extreme mobility of -the particles of air, I 

 concluded, as M. Arago had likewise done from other consider- 

 ations, that the voltaic current in air would be affected by the 

 magnet. 1 failed in my first trial, which I have referred to in a 

 note to my former paper, and in other trials made since by using 

 too weak a magnet ; but I have lately had complete success ; 

 and the experiment exhibits a very striking phacnomenon. 



Mr. Pepys having had the goodness to charge the great bat- 

 tery of the London Institution, consisting of 2000 double plates 

 (A zinc and copper, with a mixture of 1 168 parts of water, 108 

 parts of nitrous acid, and 25 parts of sulphuric acid, the poles 

 were connected by charcoal, so as to make an arc, or column 

 of electrical light, varying in length from one to four inches, 

 according to the state of rarefaction of the atmosphere in which 

 it was produced ; and a powerful magnet being presented to 



