A/A 1 IVILUAM SIEMENS, l-'.R.S. 



123 



The wires of the two bobbins are connected together, in the 

 common point a, with the pole of a galvanic battery E, the other 

 pole U-ing connected with two resistances R and z, and through 

 ihe-e with the other ends of the galvanometer-helices. The 

 resistance R is made constant, and adjusted so that when s = O 

 the index of the curve stands exactly opposite the zero of the 

 graduated scale dd\ the unknown resistance being represented by a;. 



It is evident that, the resistance in the bobbins being equal, as 

 also their dimensions and initial magnetic effects upon the needle 

 suspended between them, if we make the resistance x equal to R, 

 the currents in the two branches will be equal, and the magnet- 

 needle therefore balanced between them only when the helices are 

 equally distant from it. Should, however, either of these resist- 

 ances preponderate, the strength of current in that branch will 

 be lessened ; and in order to re-establish the balance it will be 

 necessary to shift the bobbins, approaching the one in which the 

 weaker current is circulating towards the suspended magnet. 



The instrument is erected upon a horizontal metal table stand- 

 ing upon three levelling-screws. The bobbins, with the suspended 

 magnet, and dial-plate for observing the deflection and zero of the 

 pointers are contained in a glass case with glass cover, supported 

 by four brass pillars. The instrument is supplied with terminals 

 for the battery-connexions, and a current-breaker for interrupting 

 the battery-circuit. Opposite to these are four terminal screws for 

 receiving the ends of the resistances R and x, 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, R that which is used during the measurement, and , 

 a resistance of known value, which is introduced between the 

 terminals x in order to enable the operator for his own security to 

 make a control measurement by which he may convince himself 

 of the adjustment of the instrument at any time. Another pur- 

 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 curve of this instrument, it might 

 be determined by calculation from the formula given by Weber 

 for the deflection effect of a circular current of known dimensions 



