124 DOVE ON THE ELECTRICITY OF INDUCTION. 



be made to disappear. In this manner, however, we should 

 hardly be able to judge of very slight differences, as the current 

 must have a certain amount of intensity in order to give rise to 

 a spark when the circuit is broken. I therefore pursued a dif- 

 ferent mode of experiment. On a common axis of rotation three 

 notched wheels were fixed, two of which are represented in the 

 annexed woodcut. By the side of each notched wheel, and con- 

 nected with it by a copper cylinder, is a disc, which dips con- 

 stantly into a vessel con- 

 taining mercury, whilst the 

 notched wheel is alternately 

 in and out of connexion 

 with the mercury in a simi- 

 lar vessel: thus a previously 

 existing metallic connexion 

 is interrupted. This is the 

 rheotome or current interrupter which is now so frequently used 

 for producing rapid consecutive disconnexion, which was in- 

 vented in Germany before the year 1804, and is described in 

 Aldini's Traite du Galvanisme, i. p. 202, pi. 6. figs. 2 and 5. As 

 two such rheotomes joined in an alternate manner with each 

 other, and intended to convert an alternating current into one 

 in the saine direction, have been called a commutator, 1 shall call 

 three similarly arranged notched wheels, intended to disconnect 

 simultaneouslj' two equal induced currents traversing two totally 

 unconnected Mires, a disjunctor. When all the three notched 

 wheels, which are moveable round their common axis by means 

 of screw clamps, are fixed in the same position, the first rs is 

 connected by means of the mercury vessel i, which is trans- 

 versely perforated with a cylindrical hole, with the galvanic 

 battery, the second p cr is connected with the induction-spiral 

 « /S, the third p^ a^ with the induction-spiral 78*. 



We obtain, therefore, when the spirals are empty, two per- 

 fectly equal currents in distinct wires, their compensation having 



* The disjunct! r, consisting of three notched wheels, described above, can 

 likewise be used for ascertaining what the efiect is upon an induced current 

 when it circulates in a closed wire for some time after its })i-oduction. If the 

 second and third wheels are somewhat altered in position, the disconnexion is 

 not quite simultaneous, and we can then ascertain which of the disconnected 

 wires exhibits the most powerful physiological or other action. For testing 

 the intensity of the spark quicksilver is to be preferred ; for other effects the 

 disconnexion is better effected by pieces of interposed glass. This apparatus was ' 

 constructed very carefully by M, Wagner. 



