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II. A Design for a Standard of Electrical Resistance. By J. 

 A. Fleming, M.A., D.Sc, Professor of Electrical Technology 

 in University College, London*. 

 [Plate 1.1 



IN designing a standard of electrical resistance the two 

 points to which attention is directed are the choice of 

 the material in which the standard is embodied, and the form 

 or disposition of the instrument. 



Experience is yet far from complete as to the entire per- 

 manence of wires of alloys over prolonged periods of time when 

 employed as standards of electrical resistance ; but having 

 regard to the inconveniences which attend the use of mercury 

 in standards intended to be conveyed about, evidence, as far 

 as we have it, points to the tolerable permanence of the 

 platinum-silver alloy [foQ p.c. of silver + 32 p.c of platinum) 

 when drawn into wire, for use as the material substance of 

 which the actual standard is made. 



A definite length and gauge of standard wire has then to 

 be so arranged that, whilst kept at a constant temperature, 

 currents can be passed through it and the resistance between 

 certain points ascertained. 



The form which has hitherto been chiefly manufactured, 

 and which is in most general use, is the form of standard 

 which was designed by the Committee of the British Associa- 

 tion on the original introduction of the B.A. unit, and shown 

 in plate 4 of the ' Reprint of Reports on Electrical Standards,' 

 by Prof. Fleemmg Jenkin. In this form of standard the 

 actual coil is wound on a bobbin consisting of a tube of thin 

 brass having ebonite cheeks. Attached to these cheeks are 

 the two long bent copper rods which serve as the electrodes, 

 held in position by a distance-piece of ebonite. In order 

 that the coil may be immersed in a medium of known tempe- 

 rature it is further enclosed in a thin shell of brass consisting 

 of a double tube (see fig. 1), and the whole shell filled up 

 with paraffin wax or ozokerit. Some makers then place a 

 thin lid of ebonite on the top of the shell. 



Experience gained by a rather extensive use of standards 

 of resistance of this form has indicated to the writer that this 

 edsign can be, with some advantage, modified. The disad- 

 vantages of the present B.A. form of standard are as follows: — 

 When in use the standards must be placed in water of a known 

 temperature or in melting snow or ice. After a sufficiently 

 prolonged time the temperature of this water can be taken, 

 and the temperature of the water will be the temperature of 



* Communicated by the Physical Society : read November 10, 1 888. 



