DOVE ON THE ELECTRICITY OF INDUCTION. 165 



keeper is 5" long, 2" broad, and i" in thickness. Each of the 

 four cyhnders w has a diameter of 16'"; the magnet, consist- 

 ing of four lamellae, is 10" long; the height of the four pieces 

 together is 22". The internal distance between the poles is 1", 

 the external 4|". The rotating wheel is at the side, and revolves 

 obliquely to prevent the abrasion of the crossed cord ; it can be 

 drawn out from the base of the machine, by which means the 

 requisite amount of tension can be given to the cord. At each 

 turn of the wheel the keeper revolves 8| times. The support 

 extending from 8 to 11 on the left side is 5" high; the supports 

 on the right-hand are only 2" high, by which means the side 

 view of the apparatus is better seen. The distance of the rota- 

 ting keeper from the magnet is regulated by the screws between 

 which the axis turns. The two wire coils surrounding the limbs 

 of the keeper can be connected in a twofold manner, either so 

 that the one forms a continuation of the other, or that both 

 are connected at their two extremities, so as to form a so-called 

 parallel connexion 440' in length. The changes in the intensity 

 of the resulting current which are produced when the wire is 

 coiled in a particular manner, have lately been shown by M. 

 Lenz*. For if L represents the resistance to conduction of one of 

 the coils, A the resistance to conduction of the apparatus inserted 

 for measuring the current, then with a parallel connexion there 

 are two ways presented to the current induced in the wire coil 

 at its exit, namely, the apparatus for measuring the current and 

 the other wire coil, between which it divides itself in an inverse 

 ratio to their resistance to conduction. If A therefore repre- 

 sent the electromotive force of a coil of wire, then with a paral- 

 lel connexion a current of the intensity — " will circulate 



2 A + L 



through the measuring apparatus ; if, on the contrary, the con- 

 nexion is continuous, a current will pass of the intensity -'" . 



If therefore the apparatus for measuring the current offers as 

 great a resistance to conduction as one of the electromotive coils 

 of wire, i. e. if A = L, then the parallel connexion is quite as ad- 

 vantageous as the continuous, and thei-e is no occasion in this 

 case for any arrangement to eifect both connexions. As how- 

 ever the same machine has to be used with different kinds of 

 apparatus for measuring the current, and it is not convenient to 



* Bulletin Scienliftque de I'Academie lie St. Pitershourg, \\. p. 78. 



