Kohlrausch ontheElectroscopk Properties of the VoltaicCircuit. 325 



A thin needle of silver wire, two inches in length, is suspended 

 horizontally from a glass fibre of exceeding fineness ; the fibre 

 which passes in the usual manner through a glass tube is fast- 

 ened to a torsion-head, the index of which being turned causes 

 the little needle of silver wire at the other end to follow it. The 

 needle lies across a thin strip of silver of its own length, through 

 a slit in the centre of which the needle can descend ; at the slit 

 the strip is so bent right and left, that the needle, in following 

 the index, can lay its entire length against the strip. This is 

 the only portion of the instrument which requires a drawing to 

 make it clear ; it is represented in fig. 4. AB is the strip of silver, 

 cd one-half of the needle crossing the strip in its centre, the other 

 half is hidden by the strip. AB can be raised or lowered, so as 

 to be in contact" with the needle or detached from it. When the 

 needle crosses the strip at right angles, the latter is raised so 

 that the needle rests upon it, the apparatus thus forming a con- 

 tinuous cross of conducting material. Electricity, being commu- 

 nicated to the strip, distributes itself over the entire cross; when 

 this is effected, the strip is lowered so that the needle again 

 hangs free. The index above being turned, the needle will be 

 solicited by the torsion of the fibre to approach the strip, but 

 being charged with a like electricity, it will be repelled ; by this 

 play of torsion, on the one hand, and repulsion on the other, we 

 arrive at a knowledge of the tension of the electricity communi- 

 cated. The author'has constructed tables from which the. elec- 

 tric tension due to any observed amount of torsion can be in- 

 stantly ascertained. 



In connexion with the electrometer a condenser was made use 

 of, the accuracy of which was carefully tested beforehand. For 

 experiments with the galvanic circuit, both plates are of brass, 

 suspended in a suitable frame by strings of silk, and separated 

 from each other by three little patches of shell-lac placed at 

 three different points near the periphery. AVhen the poles of 

 the battery are connected with these plates, the one becomes 

 charged with positive, the other with negative electricity ; and 

 the strength of the charge is estimated by removing one of the 

 plates to a certain fixed distance, and bringing the other, by 

 means of an isolated copper wire, into connexion with the elec- 

 trometer. 



The electromotive force of a voltaic element, which Ohm ex- 

 presses in his formula by the letter E, can be variously ascer- 

 tained : the question suggested itself to M. Kohlrausch, whether 

 any relation existed between this force and the tension of the 

 electricity at the two poles of the element. The electromotive 

 forces of various combinations were determined by Vi heatstone's 

 method. To ascertain the tension al thepolesy the circuit, which 



