424« Prof. E. Wartmann's Eighth Memoir on Induction : 



differential resistance-measurer of Professor Wheatstone is- 

 employed. The needle remains at zero under the influence 

 of the two contrary actions which affect it. It is only neces- 

 sary then to place a rheostat in the circuit of one of the wires to 

 diminish its conductibility, and give to the current circulating 

 in the other an increasing preponderance. When the index 

 has been brought to the fortieth or fiftieth degree, it detects 

 the feeblest variations in the intensity of the current. 



213. This method is inapplicable when the current is very 

 powerful. A part of its intensity would be spent in over- 

 coming the resistance of the wires of the instrument, and the 

 latter might be deteriorated. In this case we must not circu- 

 late the whole of the current through the rheometer, but only 

 introduce into it a part, the amount being in proportion to the 

 delicacy of the apparatus. 



Si*. This may be effected in two ways. Let us suppose 

 that the wires of the galvanometer terminate in two glasses 

 filled with mercury, in which are plunged the extremities of 

 the circuit traversed by the voltaic current. A conductor will 

 serve to unite these two glasses. This conductor may be a 

 rheostat, which, by changing the length of the communicating 

 wire, will determine a variation in an inverse direction in the 

 intensity of the part of the current which is conveyed to the 

 multiplier. 



215. We may also give constant dimensions to the con- 

 ductor which joins the two glasses, and connect this rheostat 

 with the rheometer. It is always possible to separate in this 

 manner a part of the whole current proportioned to the sen- 

 sitiveness of the instrument without weakening the energy of 

 the action of the battery upon the substances comprised in its 

 circuit. The measure of the variations of this energy will be 

 diminished in the relation of the abstracted to the total cur- 

 rent; but this diminution will be compensated by the perfec- 

 tion of the system of needles subjected to the influence of the 

 abstracted current, as long as the battery is not of very great 

 power *. 



§ XXI. Does Electro-magnetic Induction modify the conduce 

 tibility of bodies for Electricity ? 



216. Philosophers have carefully determined the electro- 

 dynamic relations which are established between a magnet 

 and a conductor traversed by an electric current when one of 



* This method of derivation has already been employed in my second 

 memoir (74.). See the important work of Mr. Wheatstone, entitled "An 

 Account of several new Instruments," &c., Phil. Trans. 1843, part 2,n. 322; 

 or Ann. de Ch. et de Ph., vol. x. p. 288. Third Series. 



