II. ELECTEICAL RHEOMETRY. 38 



3. The current was inverted, and the deviations of both ends of the needle on 

 the other side of the meridian observed, and their mean taken. 



4. Finally, the mean of the two preceding means was taken, and kept as the 

 deviation of the needle from the meridian line, as the base of the calculation. 



Having thus determined the deviation of the needle in the centre of the circle, 

 the current is again made to run direct, and when the needle is fixed, the telescope 

 is directed to that end of the needle which points to a division, and that point is 

 taken as a fixed one, to which the needle must always be brought. The inspection 

 of the other end is not wholly overlooked, but there is scarcely any necessity of 

 observing it. 



After this, the circle or globe is raised by turning the nut C, Fig. IV., until the 

 needle corresponds to froths of the radius. The deviation of the needle increases 

 immediately, and a few turns of the rheostat are to be uncoiled from the metallic 

 cylinder until it returns again to the former central deviation. The number of 

 turns is carefully marked, and we proceed to another experiment by raising the 

 globe or circle again until the needle is at /oths, and the same operation as before 

 is repeated, and so on until it is settled at Aths ; here it is necessary to use the 

 utmost care to take an accurate measure, since the force varies rapidly, in conse- 

 quence of the approach of the currents, and a small error in the position of the 

 needle produces a great one in the result. Besides the total resistance in this case 

 being very great, the needle is not sensible enough to small valuations of resistance. 

 This last cause, however, has a small influence on the final result. But in order 

 to diminish these errors as much as possible, we made two observations of the force 

 at /oths of the radius for each series of experiments. That is, after having mea- 

 sured it the first time, the needle was made to correspond to the centre again, its 

 deviation being reduced to the fixed mark and placed again to the /gths. There 

 was scarcely any necessity for repeating measurements for the other positions, the 

 results being always identical. 



Working thus by the direct reading of the rheostat, we have a portion of the 

 total resistance of the same circuit. Thus if R mark the total resistance at the centre, 

 the galvanometer included, x and y the turn of the rheostat at the centre and at 

 any other position, the ratio of resistances will be 



RA- X _ / 

 RT~y~T 



To illustrate all this explanation of experiments, I shall extract from the journal 

 of observations one of the experiments made on June 4, 1849, with all its details, 

 that they may be sufiiciently appreciated by the reader. 



Series of Experiments made for the purpose of determining tJie Force of a Circular 

 Current on different points of its Vertical Diameter} 



Diameter of the circle 45centm.. 



Length of the needle 4™°""- 



' As one part of these experiments was made and calculated with metrical measurements, we shall 

 therefore use them in future. 



