by the arbitrary positions ol the connecting wires 



from the test circuit (i) the 1 1 hi t i M \ i lips, but ( iiui- 



ming drew some interesting conclusions I 



measurements he made. 



Observing needle deflections for various positions 

 ol the wire A-B, with a "constant" voltaic circuit, he 

 found that "the tangent ol the deviation varies in- 

 versely .is the distance ol the connecting wire from 

 the magnetic needle" Here is a combination of the 

 deflection law for a needle in .1 transverse horizontal 

 held .ind the magnetic-force law foi a long, straight 

 wire. The latter had been determined experimentally 

 by Biot and Savart, in November 1820, In timing the 

 oscillations of a suspended magnet M 



Cumming considers liis straight-wire calibrated 

 "galvanometer" to he a device for "measuring" 

 galvanic electricity; on the other hand, his multiple- 

 loop "galvanoscopcs" are lot "discovering" galvanic 

 electricity. With the multiplier instrument, he found 

 galvanic effet is (i.e., needle deflections) using copper 

 ami zinc electrodes with several acids not previously 

 known to create galvanic action. A potassium- 

 mercury amalgam electrode created a powerful cell 

 with zinc as the positive electrode, establishing both 

 the metallic nature of potassium and the fact that 

 it is the most negative of all metals. 



In a third paper, presented April 'It'. 1823, 

 Cumming reports use of the galvanoscope in experi- 

 ments on the thermoelectric phenomena recently 

 discovered hv Seebeck. His note that "for the more 

 minute etlei ts ,i compass was employed in the gal- 

 vanoscope, having its terrestrial magnetism neutral- 

 ized . . ." seems to he the earliest mention of this 

 version of the astatic principle, a technique whose 

 dramatic effects were especially valuable in low- 

 resistance thermoclcctirc circuits, where the extra 

 resistance of additional multiplier turns largely offsets 

 their magnetic contribution. In detail, "the needle 

 is neutralized hv placing a powerful magnet North 

 and South on a line with its center: and another, 

 which is much weaker. East and West at some distant e 

 above it: hv means of the first the needle is placed 

 nearlv at right angles to the meridian, and the adjust- 

 ment is completed by the second." 



( )n varying the length of the connecting w ire of the 

 circuit. Cumming found the del], , tions nt the multi- 



*• Reported in Annates d<- Chimie et de Physique (1820), vol. 



I 5, pp 222 22 I 



"On the Development "l Electro-Magnetism 1>\ Heat," 



Transactions of the Cambridge Philosophical s 1823), vol. 



2, pp. 47 7'.. 



Figure ~. -"Si hwj iggi r mh him u r" used In < tersted in 



iHjf. A thin magnetic needle is held in a light, paper 



sling at I. suspended by a tine, vertical fiber. From 

 Annates dt Chimie et 



plier needle to be in a nearlv reciprocal relation. 

 He speaks of the "conducting power of the 

 and seems not far from visualizing Ohm's law, of 

 which no published form appeared until 1 826. ( )hm's 

 own experiments were made with very similar 

 apparatus. 



Conclusions 



An effort has been made to show- that electrical 

 experimenters prior to Oersted's discovery in 1820 

 were in desperate need of some electrical instrument 

 for galvanic or voltaic circuits that would combine 

 sensitivity, simplicity, reliability, and quick resp 



PAPER 38: EARLIEST EI Et IRnMAI.M I |i INS I Rl MEMS 



135 



