4 Mr. Donovan on Galvanometric Deflections 



rubbed in the naked hand, and in thus producing an accidental change of tem- 

 peratui-e. This explanation was offered on the assumption that friction in itself 

 is not capable of producing any effect. Between the two extremes of tribo- 

 thermo-electric omnipotence and nullity, I have tried to discover the middle 

 course of truth." 



Professor Erman then delivers his own opinion, and the facts from which 

 he has deduced it: — " A bar of bismuth was joined to that branch of the rheo- 

 phore of this instrument (the galvanometer) where the silver of a voltaic 

 element (silver and zinc) produces an eastern deviation ; and a bar of antimony 

 to the other branch of the rheophore. Both of these bars were provided with 

 handles, so that they could be employed without undergoing any change of 

 temperature in the manipulation. When, through these being stationed in the 

 same room, the two bars had previously arrived at the temperature of the sur- 

 rounding space, no deviation whatsoever was produced by their contact, but the 

 slightest friction of either of them against the other gave immediately an eastern 

 deviation. This latter extended even to an entire revolution of the needle in the 

 same direction, if the friction proceeded rather more rapidly. By gently raising 

 the temperature of the two bars to 30° or 3.5°of Reaum., scale (100° or lll°Fahr.), 

 their contact in a state of repose always produced a stationary eastern deviation 

 of about 30°, which by rubbing was further increased to 60°, and there likewise 

 remained invariable as long as friction continued. At length, when I cooled 

 the bars below the temperature of the room, by the evaporation of naphtha 

 vitrioli (sulphuric ether), their contact continually produced a western deviation, 

 which by rubbing was instantaneously changed into a contrary or eastern one 

 of apparently the same amount as before, and this likewise remained stationary 

 as long as the friction continued; but, by the interruption of it the western 

 deviation was immediately restored. This simple sketch of the phenomena of 

 changes of intensity or even of sign, which friction, at the point of contact, 

 gives to the deviation of a multiplicator's needle, will already suffice to exhibit 

 it as a mere consequence of the heat produced by the action of rubbing."* 



Professor Erman, in alluding to the difficulty of investigations of this kind, 

 and, as he expresses it, " the arduous nature of the observations required," 



• Report of Fifteenth Meeting of British Association, June, 1845, p. 103. 



