398 Intelligence and Miscellaneous Articles, 



If from time to time we make a comparative measurement at once 

 with an electrometer and a tangent-compass in the preceding con- 

 ditions, the numbers given by the compass, multiplied by a coeffi- 

 cient, will show the difference betM'een the tensions corresponding 

 to the extremities of 1 kilometre of wire ; we shall thus have the 

 exact value of the total resistance of the circuit, if the elements em- 

 ployed are fresh. That value will only be approached if the electro- 

 motive force of the pile has diminished. 



The use of the electrometer for the determination of the intensities 

 of currents is known ; the results obtained at different periods are 

 comparable if we are careful to measure at the same time the differ- 

 ence of the tensions at the two poles of a Daniell's element. 



The Daniell's element gives constant differences between its two 

 poles when it is constituted in the following fashion : — a vessel con- 

 taining a plate of amalgamated zinc and a saturated solution of sul- 

 phate of zinc ; a second vessel with a saturated solution of sulphate 

 of copper and a plate of copper : a U-tube containing sulphate of 

 zinc, and closed at its extremities with gold-beater's skin, puts the 

 two vessels in communication. 



In the formula 2 = C » let C = l ; the current having the unit 



of intensity will be that for which the difference of the potentials at 

 the two extremities of the unit of resistance is equal to the unit of 

 electric potential. 



The unit of potential is the potential of a sphere, charged with 

 the unit of electricity*, whose radius is equal to the unit of length. 



If we take for the unit of intensity the intensity of the current 

 which produces at the two extremities of 1 kilometre of telegraphic 

 wire a difference of tension equal to one hundredth of that which 

 exists between the two poles of a Daniell, the intensities of the cur- 

 rents in the above-reported experiments will be 146, 61'Q, 40-34, &c. 



But, in order to give more generality to the definition of the unit 

 of intensity, it is expedient to value in electrostatic units the poten- 

 tial at one of the poles of a Daniell's element when the other pole is 

 connected with the earth. 



If the pole of the pile is put in communication by a long wire with 

 a sphere of radius 1, this sphere takes the same potential as the pole 

 of the pile, and the quantity of electricity on its surface expresses 

 the potential of the pole. 



The measurement was made with a torsion-balance, which was 

 constructed as follows. A moveable gilt ball was fixed at the extre- 

 mity of a rod of gum lac which was supported by a torsion-thread ; 

 this last was in metallic communication with the ball. A fixed ball, 

 of the same diameter as the other, was borne by a stick of gum lac ; 

 and a fine thread connected it externally with the prolongation of the 

 torsion-thread. The centres of the two balls were at first placed at 

 a certain distance (about 3 centims.) without there being any torsion. 



* The unit of electricity is the quantity \\hich, acting upon an equal 

 quEDtity placed at the unit of distance, viz. 1 millimetre, produces a repul- 

 sive force of 1 miUigramme. 



