ON STANDARDS OF ELECTEICAL RESISTANCE. 481 



will be equal, and the magnet-neecUe therefore balanced between them only 

 when the heUces are eqiially distant from it. Should, however, either of these 

 resistances preponderate, the strength of current in that branch will be les- 

 sened ; and in order to reestablish the balance it will be necessary to shift the 

 bobbins, approaching the one in which the weaker current is cii'culating towards 

 the suspended magnet. 



The instrument is erected upon a horizontal metal table standing upon 

 three levelling-screws. The bobbin, the suspended magnet, and dial plate for 

 observing the zero of the pointer are contained in a glass case, supported by 

 four brass pillars. The instrument is supplied with terminals for the battery- 

 connexions, and a current -breaker for interrupting the battery- circuit. Oppo- 

 site to these are four terminal screws for receiving the ends of the resistances 

 11 and .V, with contact-plugs between them, in order to quickly establish a 

 short circuit in case the operator should be in doubt towards which side he 

 has to move the adjusting-curve. Two constant resistances accompany the 

 apparatus E, — that which is used during the measurement, and a, a resistance 

 of known value, which is introduced between the terminals .v in order to 

 enable the operator for his own security to make a control measurement by 

 which he may verify the accuracy of the instrument at any time. Another 

 pui'pose of this resistance is to facilitate the readjustment of the zero-point, in 

 case the galvanometer should at any time be cleaned or a new silk-fibre put in. 



In constructing the sliding ciu've of this instrument, it might be determined 

 by calculation from the formula given by Weber for the deflection produced by 

 a circular current of known magnitude upon a magnetic point, and from the 

 given distance of the coils from each other. I prefer, however, in practice to 

 determine the curve of each separate apparatus empirically, because it is not 

 possible to coil a helix mathematically true, or to set it, when coiled absolutely 

 at right angles to the plane of its horizontal motion. 



In the determination of each curve I use a deHcately adjusted rheostat or 

 scale of resistances in the circuit of x, giving it varying valiies corresponding 

 to the equal divisions of the engraved scale, and constructing the curve accord- 

 ing to the position which it is found necessary to give to the point s' in order 

 to arrive at the magnetic balance. "With each instrument it would be possible 

 to have two values of E. — one expressed in mercury and the other in B. A. units ; 

 and in order to measure at pleasure in either of these units, it would only be 

 necessary to insert the one or other between the terminal screws for R. 



The instrument has been found to be very convenient for the measirrement 

 of the wire -resistances of overland lines, or for the reading of resistance ther- 

 mometers ; it reduces the operation and the observation of the zero position of 

 a needle, and the reading upon a graduated scale, which can be performed by 

 a person of ordinary intelUgence without experience in electrical measurement. 

 In accuracy and range it equals the bridge method, while as regards portability 

 and cheapness of apparatus the advantages are decidedly in its favour*. 



II. 0)1 a Modification of Siemens' s Resistance-Measurer. 

 By Fleeming Jenkin, F.E.S. 

 The following method of measuring resistances was suggested to Mr. Jenkin 

 by the above invention of Mr. Siemens : — 



Let two tangent galvanometer-coils of equal magnetic moment be fixed 



* I have lately constructed the same instrmnent on this principle with a circular instead 

 of a straight sliding-pieee, which gives the advantage of a longer graduated scale in tho 

 form of a circle. The circular sliding curve is adjusted by radial set screws in a solid ring 

 working in a V-groove round the galvanometer. 



1867. 2 K 



