ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



145 



Fig. h. 



it must not be annealed in free air, because the manganese then would 

 oxidise, and the qualities of the material would be altered. Thus it is 

 not possible to buy, for instance, a ^vire, say 1 mm. thick, and to draw it 

 dowa to the required gauge without taking proper precautions. 



In concluding, I will very briefly refer to the construction of our 

 wire-standards — for instance, to a standard of one ohm (fig. 2).' 



The double silk-covered wire is wound on a metal bobbin, h b, which 

 is previously covered with a thin piece of silk, coated with shellac var- 

 nish and heated, in order to secure good insulation. The bobbin can be 

 screwed to the ebonite disk d, but it is not fixed to it before the accurate 

 adjustment of the resistance. The resistance of the wire must be 1-2 per 

 cent, larger than one ohm to begin with ; then it is wound on the bobbin, 

 heavily coated with shellac varnish, and heated in an air-bath at a 

 temperature of 140° C. during about five houi-s. By this procedure we 

 obtain, as already stated, very constant 

 resistances ; further, the shellac is melted 

 at this temperature, and becom.es after 

 cooling a hard, highly insulating mass, 

 which at the same time protects the 

 wire against any chemical action. 



To the ends of the wire are previously 

 soldered with silver two small copper 

 rings. The exact adjustment is made 

 by means of a fine wire-resistance,/, of 

 100-200 ohms put in multiple arc with 

 the thick wire. A comparatively great 

 length of this fine wire corresponds to 

 a' very small change of the whole resist- 

 ance, and so it may be easily adjusted to 

 a few thousandths per cent. Then the 

 small rings at the ends of the two wires 

 are screwed together and soldered to 

 the stout connecting pieces, pp. A wide 

 brass case, c c, serves to protect the wire. 



In taking observations the resistance 

 is put in an oil-bath (fig. 3) ; the tem- 

 perature of the wire may then easily be 

 determined, and besides that, there can- 

 not exist any thermo-electric force be- 

 tween the two solderings. It is a matter 

 of fact that the thermo-electric force of 

 manganin against copper is very small 

 indeed; it amounts only to 1"5 micro- 

 volt for 1° C. ; the corresponding value 

 for other resistance materials is generally 20-30 microvolts. We see that 

 even in this respect the manganin is much preferable. 



The construction of standards of O'l ohm, and, on the other hand, of 

 10, 100, 1,000 ohms and more, is essentially the same as described. Of 

 course there is no multiple arc to those of 10 ohms and more. 



As to the constancy of manganin resistances 1 will quote a few 

 figures. Table VII. refers to a resistance which is used to determine 



1892. 



See Dr. K. Feussner, Zeitsctvrift fiir Instrumentcnhunde, 1890, p. 6. 



