II. ELECTRICAL RHEOMETRY. 35 



galvanometer, and from this to the rheostat, Fig. III., then through the coils RR' 

 and afterwards to the globe, and returns back to the zinc Z. 



The detailed description of these things, although tiresome, is by no means 

 superfluous, since too much care cannot be taken in such complicated experiments, 

 if we wish to avoid error. We shall see that, notwithstanding all this care, much 

 more is yet to be done in order to make these experiments such as the importance 

 of the subject requires. 



§ 9. Description of the Experiments and Results obtained with the Circle. 



The formula to be verified in order to find the intensity of action of a circular 

 current on a needle placed in the different points of the vertical diameter OX of 

 a circle or globe, is given in § 7, and is 



h f{d,p) p ■ 



We must therefore determine the absolute resistance of the current when the 

 needle is in the centre, and then when it is placed in any other point of the vertical 

 diameter. The partial resistances which compose this total resistance are: 1st, the 

 battery; 2d, the galvanometer; 3d, the rheostat; 4th, the wire surrounding the 

 globe or the circle. In order to avoid many measurements, and the errors insepa- 

 rable from them, we shall call the constant resistance of the circuit the four parts 

 formerly mentioned. The rheostat is supposed to be at zero turns, that is, when 

 there is only so much of its wire in the circuit as is sufficient to connect both 

 cylinders, which will be about 4 or 5 inches. 



I. Method for determining the constant Resistance of the Circuit. — The battery 

 having been prepared and set in action some time before, the deviation of the 

 galvanometer is carefully observed for the purpose of ascertaining its constancy. 

 When it is perfectly stationary, I take a copper coil r, Fig. II., of a known 

 resistance, and divide the current between it and the galvanometer, putting the 

 ends a, h of the coil in the small cups a, h, full of quicksilver, in which the ends 

 of the wires of the galvanometer are also immersed. Two ways being thus 

 opened to the current, the deviation diminishes, and when the needle is still, 

 its point of rest is carefully marked. If the needle is not on any well-marked 

 point of the graduation, it is easily reduced to it by means of the rheostat, taking 

 note, however, of the turns so introduced. The needle being perfectly still, the 

 deviation is noted again. The coil r is taken away, the needle rises again, but 

 coiling the rheostat wire on the wooden cylinder, its strength is easily reduced, 

 so that the needle points again to the same division as before, when the current 

 was divided. Now the rheostat is observed, and the number of turns noted. 

 The constant resistance R will be given by the following formula :* — 



' The principles from which formula (1) is deduced are expressed by Wheatstone in the beautiful 

 memoir which we have already quoted; and several other papers on this subject may be seen in the Archives 



