246 Prof. E. "WaYlmann'sJourth Memoir on Induction. 



we disconnect the extremities .2? and y of the wire B from the 

 springs of the rheotrope. 



124>. Lastly, if we propose to employ the reaction of the 

 induced wire B on the inductor A, and that of the inductor 

 wound in a helix on itself, we substitute for the wire a the body 

 which is to be subjected to the effects of these reactions. 

 We then employ one of the four arrangements above described 

 (123.), according as we wish to obtain the totality of influence 

 of the two currents induced in the same direction, or in di- 

 rections alternately opposed, or again, the separate influence 

 of the direct or the inverted currents. The simple induction 

 of the inductor on itself is obtained with a reel with a single 

 wire in place of the conductor a, and the arrangement de- 

 scribed (122.). 



125. It remains for me to give some details on the con- 

 struction of the rheotrope. The three metallic wheels r, 5, t 

 (fig. 5) present on their periphery twelve hollows filled in with 

 hard wood. These heterogeneous wheels have been worked 

 together by the lathe ; they are each C^'SO in diameter, and 

 0'^"Q6 in thickness. A metallic tooth of the middle wheel 

 s exactly corresponds to one isolating part of the extremes r 

 and t. They are placed on the same brass axis /cl, v/hich is 

 turned by a winch ti or a tooth-wheel. The spring / and the 

 wheel t are in metallic contact with the axis. The wheels r 

 and 5 are, on the contrary, each isolated from it by an ivory 

 ring covered externally with a brass cylinder. These two 

 cylinders bear the wheels, and are constantly pressed by 

 springs g^ /i, which embrace them on a semi-circumference. 

 The three springs^, ^, i terminate on the three heads ^>', h', i', 

 by means of which they can communicate together. Lastly, the 

 six springs a,b,c,cl,e,J'iire made of plates of hammered copper; 

 they are fixed to the base of the instrument by screws, ?',s',^', 

 the heads of which, similar tog', and pierced like them with 

 two holes, can receive the metallic wires intended to establish 

 a connexion between the different wheels. These springs are 

 cleft in order that the groove may facilitate the adjustment of 

 their length. Above they bear a screw (fig. 6) in the part 

 which has to rest on the circumference of the wheels ; the 

 opposite notch allows of regulating the elasticity of the spring 

 and the degree of friction. The play of these pieces may thus 

 be regulated with minute precision. 



126. If it is not wanted to impart the same direction to the 

 two induced currents, the apparatus may be simplified by 

 giving it only two wheels. One is reserved to render the cur- 

 rent of the battery intermittent; the other is joined to the in- 

 duced wire; and according as there is coincidence or alter- 



