312 LOCALIZED ELECTRIZATION. 



electrodes receive the current from the thick wire, and two others, 

 which communicate with the extremities of the fine wire, receive 

 the current of the entire helix, which is then composed of a thick 

 wire in its first part, and of a longer and very fine wire in its 

 second. Every one will understand that these electrodes can only 

 give an extra current proceeding from the same coil, and that the 

 current of the entire coil will be more intense than that of its first 

 portion ; but that these currents cannot possess the differential 

 properties of the extra current and of the induced current. Such 

 instruments are sent into the market as if they possessed both. 

 Their manufacture is, as may be imagined, more simple and less 

 costly than that of double-induction instruments ; but the purchaser 

 is deceived. It is fortunately easy, when forewarned, to discover the 

 fraud, by the absence of the difterential properties of the currents. 



B, — Intermissions. — Formerly, the electro-dynamic instruments 

 made only rapid intermissions, obtained either by wheels analogous 

 to that of Masson, or by tremblers constructed on the principle of 

 the apparatus of Neeff. 



At present, most of the instruments afford, at pleasure, either 

 slow or very rapid intermissions. But each of the slow intermis- 

 sions should be perfectly isolated, distinct, and without vibrations. 



The slow intermissions of the large electro-medical apparatus of 

 Euhmkorff (see fig. 79) are deceptive; each tooth of his star- 

 shaped wheel receives a series of very rapid intermissions from 

 his trembler. I have pointed out this fault of construction to this 

 skilful maker, who has undertaken to correct it, and to produce 

 on his wheel d(3fined and isolated intermissions. 



The intermissions produced by the contact of a metal plate, 

 forming a spring, are not always well defined. The friction gives 

 one strong intermission, followed by a series of extremely rapid 

 small vibrations, which acutely excite the sensibility, and render 

 muscular faradization painful. 



The rheotomes of slow intermission have the inconvenience that 

 they either occupy one of the hands of the operator or require the 

 presence of an assistant ; with the pedal rheotome, such as I use 

 myself (see Y, fig. 52), this inconvenience is avoided. It is to be 

 desired that all makers would attach this accessory to their electro- 

 dynamic instruments. 



The greater part of the tremblers of electro-dynamic instruments 

 give vibratory intermissions. M. Gaiffe has especially endeavoured 

 to remedy this defect. 



C. — Graduation. — When I commenced my researches upon the 

 improvements that could be made in induction instruments, MM. 

 Breton freres had already constructed, under the direction of 



