ON LIVING MALAPTERURUS. 395 



when it is at its acme. Faraday attained this originally in the 

 simplest manner, by fastening- a file to each of the saddles on the 

 Gymnotus, and rubbing- one file along- the other. He afterwards 

 made use of a toothed-wheel, the circumference of which was rubbed 

 by a spring l . I imitated this mode of experiment, at first by making 

 use of a Savart's wheel of 400 teeth, turned by hand 3-4 times in 

 a second. Then, in order to be relieved of the trouble of turning, I 

 took a small toothed- wheel of 50 teeth, which was turned 9-10 

 times in a second by clock-work, constructed by Herr Gruel, for 

 experiments with colour discs 2 . The spark due to the shock of the 

 Malapterurus was perceived tolerably often in both ways, better 

 however, as far as could be judged, with the Savart's wheel than 

 with the smaller wheel. Perhaps this resulted from the 3-4 times 

 greater number of interruptions with the Savart's wheel ; more 

 probably, according to the principle stated above, because the 

 slighter contact of the spring with the teeth and the inferior con- 

 duction in the axle-bearings in the case of the small toothed-wheel, 

 weakened the shock more than with the Savart's wheel. 



As the frog-interrupter with a sufficient overweight breaks the 

 shock at about the middle of its course, it ought to furnish a new, 

 and, as it seems, the surest mode of observing separation sparks. I 

 have not yet pursued this thought sufficiently. In the few experi- 

 ments carried out with this object, the place between the point and 

 the plate on which it rests is, in the present form of the interrupter, 

 so unfavourable for observation that no conclusion can be drawn 

 from their failure. 



4. Induction. 



If a coil of 720 turns of copper wire i-i mm. thick, the interior 

 of which was 35 mm. wide, and was filled with soft iron wires, was 

 put into the circuit, the extra current naturally made the sparks 

 much stronger. 



Induction in a secondary coil by the shock of the fish had never, 

 so far as I know, been before noticed, and it gives the oppor- 

 tunity for an instructive observation. When the shock was sent 

 through the primary coil of a E/uhmkorfPs inductorium, the 

 divisions of which were connected so as to have half the length and 

 double the thickness, an observer, placed between copper handles in 

 the circuit of the secondary coil, received a shock. If the platinum 



1 Loc. dt. p. 7. 1767. 



3 Poggendorff's Annalen, etc., 1848, vol. Ixxv. p. 525. 



