272 Mr. C. W. Siemens on a Resistance-measurer. 



bin-slide is pressed against it by means of a spring, the bobbin 

 follows it in all its movements. 



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 being connected with two resistances R and x, and through 

 these with the other ends of the galvanometer-helices. The 

 resistance R is made constant, and adjusted so that when x=0 

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

 duated scale dd 1 . 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 reestablish 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 secu- 

 rity to make a control measurement by which he may convince 

 himself of the adjustment of the instrument at any time. Another 

 purpose 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 

 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 pos- 

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

 absolutely at right angles to the plane of its horizontal motion. 



