IP 



HH 





Figure 6. — Electromagnetic instruments of James Cumming, used at Cambridge in 

 182 1. One is a single-wire ''galvanometer." following Ampere's definition. Cumming 

 called the multiple-turn construction "galvanoscopes." He showed how to increase 

 their sensitivity by partial cancellation of the earth's magnetism at the location of the 

 compass needle. (From Transactions of the Cambridge Philosophical Society, vol. 1. 1821.) 



noting that the sides of a circuit produce additive 

 effects on a needle, he comments that a flattened 

 rectangular loop produces nearly quadruple the 

 effect of a single wire. The paper is primarily a 

 review of Oersted's work, with references to electro- 

 magnetic observations before Oersted, and accounts 

 of various related but nonmultiplier experiments that 

 Cumming has made. His second paper, of May 

 2 1 st, contains a fine plate (fig. 6) illustrating ar- 

 rangements used in investigating the subject of the 

 paper's title "The Application of Magnetism as a 

 Measure of Electricity." (Neither Poggendorf nor 

 any of his commentators ever illustrated his "con- 

 denser. ") 



Although this plate is never referred to in the paper 

 itself, .1 nearb) "Description" gives a few comments. 



The two wire patterns shown are noted as simply 

 "forms of spiral for increasing the electromagnetic 

 intensity." The mounted wire loop, with enclosed 

 compass needle and terminal mercury cups, is clearly 

 identical in principle with the devices of Schweigger 

 and Poggendorf, and is called a "galvanoscope." 

 The largest structure illustrated does not involve the 

 multiplying effect. It is called a "galvanometer," 

 consistent with Ampere's definition of that word. 

 To use it, two leads of a voltaic circuit arc inserted 

 into the mercury cups AC and BD, and the board 

 EFGH carrying the cups is moved vertically until 

 some "standard" deflection is obtained on the compass 

 needle below. The relative "strength" of the circuit 

 is then given by the calibrated position of the sliding 

 section. Uncertainties arc undoubtedly introduced 



II 



mill. I IN 2KI: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



