Thermo-Electricity and Electro-Magnetism. 311 



Art- VI. — Notices on Thermo-Electricity and Electro-Magnet ism 9 

 in a letter to the editor , from Prof. John P. Emmet, dated, Uni- 

 versity of Virginia, May 8th, 1834. 



Sir, — I have been induced to offer the following brief observa- 

 tions, and to request their publication, in order that 1 may have it in 

 my power to make a timely correction of a statement made in my 

 former communication " upon caloric, as a cause of voltaic currents." 

 This opportunity will likewise enable me to announce a very inter- 

 esting law of thermo-magnetism, which I altogether omitted to notice 

 in that communication. I shall also, in conclusion, be able to offer 

 to the medical portion of your readers, a notice respecting my form 

 of the coil-magnet, which I think promises fair to become a substitute 

 for the leyden jar and common electrical machine, in all such cases 

 as require the sanative agency of the latter instrument. 



The results of my communication, above referred to, and which 

 may be found in Vol. xxv, No. 2, of this Journal, were obtained 

 by means of a galvanometer, delicate it is true, but far from being 

 perfect ; and which did not indicate currents of low intensity. Short- 

 ly after the manuscript was forwarded, I constructed a multiplier of 

 excessive delicacy, and which, in all its details, exactly resembles 

 and may be understood from the instrument which I see described in 

 the last number of your Journal. I was not a little struck by the co- 

 incidence, and pleased to see the notice.* The object which I had in 

 view, was to give the maximum effect with the smallest current, and, 

 as there is always a great loss of power when the coil of the multi- 

 plier is extensive, I limited the wire to a (ew turns over and under a 

 couple of connected needles, rendered perfectly astatic. With a view 

 also, of applying the current as advantageously as possible to the 

 needles, the coil, instead of being wound upon the same spot, as usu- 

 al, was spread out, laterally, so as to form a kind of box within which 

 the lower needle traversed. By this arrangement, the tangential mag- 

 netic force of the voltaic current was applied close to the extremities 

 of the needles in every portion of their revolution. The needles 

 were suspended by raw silk, and the whole instrument included with- 

 in the glass frame of a Coulomb's balance of torsion. The delicacy 

 of this multiplier is so great that a declination of 90° may be obtained 



* By Dr. Locke of Cincinnati ; see Vol. xxvi, p. 103 



