Prof. Rijke on some Properties of the Induced Current, 257 



method, by means of a small mirror which follows the motion 

 of the helix. 



In this instrument the resistance of the air and the cm*rents 

 induced by the terrestrial magnetism are the only forces which 

 tend to deaden the oscillations of the moveable helix when a 

 momentary current has caused it to deviate from its position of 

 equilibrium. It follows that the helix only returns to this posi- 

 tion after having made a great number of oscillations. The 

 means by which the amplitude of the oscillations may be appre- 

 ciably reduced in a sufficiently rapid manner are readily ima- 

 gined ; but to extinguish them completely, much more time is 

 necessary. If each time, before commencing a new experiment, 

 it were necessary to wait until the helix was quite at rest, succes- 

 sive observations would be separated by considerable intervals of 

 time. This difficulty may, however, be easily surmounted. In 

 fact let us suppose that, while the bobbin makes its backward and 

 forward motion, the inducing circuit is opened at the moment 

 at which the bobbin passes by its position of rest, and let us 

 suppose that we obtain a deflection to the left, equal, for 

 example, to u. If care has been taken to note the elongation to 

 the right, u, which has preceded the moment at which the induced 

 current was produced, the velocity of the bobbin at the moment 

 at which it passed its position of rest would be proportional to 



u— u 1 e~ nT f 



e nT being the quantity of which the logarithm represents the 

 logarithmic decrement of the system. It is clearly easy to com- 

 pare the elongations obtained ; it is enough to subtract the quan- 

 tity u l e~ nT l . 



It is necessary then to know the value of e~~ nT ; but it is useless 

 to determine it with a high degree of accuracy, seeing that the 

 quantities u x which entered into my experiments were always 

 very small. To obtain it I employed the method of M. Weber 

 (Electrodynamiscke Maasbestimmungen, p. 273), and which is the 

 same as that which Gauss has given in the Result ate des Mag- 

 netischenVereins im Jahre 1837; and I convinced myself that so 

 long as u Y did not exceed five divisions of my scale, we could put 



e- nT = l. 

 In the experiments which I am about to describe, the ends of 

 the wire of the dynamometer were in metallic connexion with the 

 two ends of the secondary helix of a RuhmkorfFs apparatus, 

 large size. The current was furnished by four DanielPs ele- 

 ments, the zinc plates of which were 20 centimetres in height 

 and 16 centimetres in breadth. The rupture of the primary 

 current was made alternately in air and in a coal-gas flame. 

 The results I obtained are given in the following Table : — - 



