484 M. K. Kohlrausch's Theory of the 



wive of the sine-electrometer S, which was 3 feet long, was con- 

 nected with this jar by letting one extremity into a shallow cavity 

 made in its knob. As long as the jar contains no electricity, the 

 magnetic needle of the instrument stands in the meridian, making 

 an acute angle « with a revolving metallic arm. At the moment 

 of imparting a charge to the jar, the electricity would be carried 

 to the needle and arm, and the formei', being forcibly repelled 

 from the latter, would be set in violent oscillation ; but before 

 the needle came to rest, and before the angle of observation a, 

 could be again produced by turning the arm, exactly the most 

 important time for observation would be past. The object there- 

 fore was to bring the electrometer into such a condition, before 

 any electricity was imparted to the jar, that observations might 

 commence from the moment when, by falling, the lever effected 

 the charge. This is not difficult if we know beforehand the 

 angle </> through which the magnetic needle will be deflected 

 from the meridian by electric repulsion, in order that it may 

 make an angle « with the repelling arm, in other words, pro- 

 vided we already know what charge the jar will receive. Con- 

 sequently if mn (fig. 1) is the position of the magnetic needle in 

 the meridian, ab the line of vision which by means of reflexion 

 is placed at right angles to the needle, andjy^' the arm which is 

 connected in a fixed manner with the line of vision, the above 

 object will be effected by turning the latter, and with it the arm, 

 through an angle <^, and then, by means of a neighbouring gal- 

 vanic current, deflecting the needle until it is again in its proper 

 place, i. e. perpendicular to the line of vision. To this end the 

 current is made to traverse a multiplying coil M, and moderated 

 in the requisite manner by means of a rheostat or rheochord R. 

 At the moment when, by the fall of the lever, electricity is con- 

 veyed from the battery to the single jar, tlie knob of shell-lac e 

 breaks the circuit. The mechanism necessary thereto is very 

 simple. One pole of the circuit is led to the strong wire h fixed 

 to a board, the other to a spring-wire g (in the foreground of the 

 drawing), which being properly bent requires only to be hooked 

 on to Ji in order to close the current. When the knob of shell- 

 lac strikes the end/" of the wire g, it immediately interrupts the 

 current, whose deflecting force will now be replaced by the repul- 

 sive force of the electricity which has entered the electrometer. 



In order to know beforehand what charge the jar under exami- 

 nation will receive from the battery, the experimenter must 

 already know in what proportion the electricity divides itself 

 between these two bodies. In Ajjpcndix II. a better method for 

 determining this proportion will be given ; for the present the 

 following method may suffice, which, for reasons there given, 

 will always admit of sufficient exactitude when the whole coating 



