40 On a new Method of measuring the [July, 



communicates with the ground. Its upper extremity may be 

 connected by a metallic wire with an insulated vertical pivot, 

 carrying a weakly magnetic needle, balanced horizontally. On 

 a level with the needle, and distant from the pivot, about half 

 the length of the latter, is a metallic ball, also insulated, but 

 communicating with the pile. It is evident that by this 

 arrangement, the electricity accumulated at the upper pole of 

 the pile, is communicated to the needle and the ball, and con- 

 sequently repulsion ensues, tending to separate the needle, 

 which is moveable, from the ball which is stationary. If we 

 place the pivot and the ball in the magnetic meridian, the needle 

 touches it, and remains at rest as long as the apparatus is not 

 connected with the pile ; but the instant the communication is 

 established between them, the needle is repelled, and after some 

 oscillations takes its position of equilibrium, depending on its 

 magnetic power and the energy of the pile. These two- quanti- 

 ties remain constant for a considerable time, with the same appa- 

 ratuses maybe ascertained, by determining the angle which the 

 needle makes with the magnetic meridian, after it has assumed 

 a fixed position, by means of a divided circle adapted to the cage 

 which covers it. A simple conducting needle suspended by a 

 metallic wire of proper diameter and length, might be substituted 

 for the magnetic one ; but M. Rousseau's apparatus is much 

 more convenient, and sufficiently sensible for the kind of effect 

 which it is his object to measure. 



To use it for ascertaining different degrees of conducting 

 power, it is sufficient to place the substance submitted to expe- 

 riment in the electrical current, taking care that the thickness 

 which the electricity has to pass through be always equal. If 

 the flow of the quantity of electricity necessary to produce the 

 greatest deviation be not instantaneous, the time required by the 

 needle to assume its fixed position, may be taken as the measure 

 of the degree of the conducting power of the substance em- 

 ployed. 



To submit liquids to this kind of examination, M. Rousseau 

 places them in small metallic cups, communicating by their 

 foot with the needle and the ball : he then places in the liquid 

 one of the extremities of the metallic wire, covered with gum 

 lac, that the same surface of metal may always be in contact 

 with the fluid, and measures the duration of the needle's motion 

 from the moment when the communication is established with 

 the pile by the other extremity of the wire. 



By submitting the fixed vegetable oils employed in the arts 

 and in domestic economy to this kind of proof, M. Rousseau has 

 established a very singular fact, which may be useful in com- 

 merce ; it is that olive oil possesses a very inferior degree of 

 conducting power to that of all the other vegetable or animal 

 oils, which nevertheless present, in all their physical proper- 



