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XV. On the Use of DanielVs Cell as a Standard of Electro- 

 motive Force. By J. A. Fleming, M.A., D.Sc. (Lond.), 

 Fellow of St. John's College, Cambridge, Professor of Elec- 

 trical Technology in University College, London *. 



[Plate V.] 



THE extensive use now being made of strong currents of 

 electricity for purposes of illumination necessitates the 

 employment of instruments for measuring current-strength 

 and electromotive force of high values. The employment of 

 electricity for telegraphic purposes, and especially in sub- 

 marine telegraphy, compelled attention to a common accuracy 

 in measurement of resistance and capacity; but for these 

 purposes the measurements of electromotive force and currents 

 are not of such importance as they are in the recent develop- 

 ments. This requirement has drawn the attention of electro- 

 mechanicians to the subject, and led to the introduction of 

 numerous forms of electromotive force and current- indicators, 

 variously called potential galvanometers, voltmeters, pres- 

 sure-gauges, ampere- or am-meters. 



Considering first the voltmeters, they may be classified, 

 according to Sir W. Thomson's nomenclature, into two classes 

 — idiostatic and heterostatic. In the second class the magnetic 

 force at a point in the neighbourhood of a conductor con- 

 veying a current is compared with another magnetic field 

 either of the earth or of a permanent magnet, and the differ- 

 ence of potential at the ends of the conductor deduced there- 

 from. In the first class no magnetic field is made use of other 

 than that created by the current itself. 



In the case of heterostatic instruments the readings require 

 to be controlled by a special determination of the value of the 

 auxiliary field; and in the case of an ordinary tangent- or sine- 

 galvanometer, the determination of the earth's horizontal 

 magnetic force is a task requiring great care and considerable 

 precautions. In the idiostatic instruments, in many cases, 

 springs are used to obtain a force with which to compare the 

 magnetic force in the neighbourhood of a conductor. The 

 elasticity of these springs is not necessarily permanent, and is 

 liable to change by temperature as well as time. Accordingly 

 in these instruments also, the constant must from time to 

 time be determined. Pending the appearance of the new 

 instruments promised by Sir W. Thomson, in which no 

 springs nor permanent magnets are used, it is necessary to 

 fall back upon galvanic cells to establish a standard of elec- 



* Communicated by the Physical Society: read June 27, 1885. 



