HISTOEY, &c., OF INDUCTION INSTRUMENTS. 313 



Rognetta, an electro-dynamic apparatus of double induction, in 

 which the graduation of the induced current was obtained by- 

 gliding the coil of thick within the coil of fine wire. While 

 gradually placing tiie first within the second, the induction only 

 occurred in the spirals that were brought into mutual relation, 

 and hence the intensity was proportionately increased. The reverse 

 occurred as the first coil was withdrawn. This method of gradua- 

 tion was perfect ; and I originally adopted it for my volta-electric 

 and magneto-electric instruments (see fig. 51) ; modifying it, 

 however, in an important manner, and applying it to the gradua- 

 tion of both coils (which Rognetta had neglected because he did 

 not utilize the extra current, — not being aware of the difference of 

 its action from that of the second coil). 



Instead of the internal coil (of thick wire), as in the apparatus 

 of Rognetta, it is the external coil of fine wire which I have ren- 

 dered movable, and which, in covering or uncovering more or less 

 of the internal, glides upon two metallic conducting stems, which 

 are in communication with the extremities of its wire and with the 

 knobs of the electrodes. In order to obtain the graduation of 

 the extra current by the second coil, I completed the circuit of its 

 wire, and caused it to move over the first coil. It is needless to 

 say that the graduation was the same as in Rognetta's instrument. 



This system of giaduation was excellent, but I have abandoned 

 it, since I discovered that metallic cylinders moving over the in- 

 terior and the exterior of the coils graduated equally well the 

 intensity of the currents. Then, instead of constructing two inde- 

 pendent coils — the external to receive the internal wliich remained 

 fixed, — I wound the coil of fine wire directly over the thick wire 

 (each layer being insulated, it must be understood, by varnished 

 paper). The result was, an increase of the intensity of the induced 

 current, by the approximation of the two coils, so that it equalled 

 one produced in two thousand metres of wire ; and a simplification 

 of manufacture. My instrument had also gained in solidity. 



Graduation by the mutual influence of the coils (which, it will 

 be seen, is of French origin) has been adopted by M. Ruhmkorff, 

 who has constructed what is known as the carriage apparatus of 

 M. Du Bois-Reymond. 



It was, doubtless, in order to please some admirers of everything 

 that is done at Berlin, that M. Ruhmlvorff has imitated the ap- 

 paratus of Siemens in his large medical instrument. (I shall 

 hereafter show the inconveniences of this system of graduation, 

 especially in Ruhmkorff's instrument.) The preference given by 

 most of the Paris manufacturers to my system of graduation by 

 metallic cylinders, proves that I had reason for my choice. 



