DOVE ON THE ELECTRICITY OF INDUCTION. 185 



on breaking connexion in a wire through \vhich an electric 

 current is circulating, occurs at the moment connexion is broken, 

 or at a measurable time after this interruption. This occurs in 

 Saxton's machine, when the sliding spring leaving the metal 

 comes in contact with wood, which happens in a certain position 

 of the keeper. If the spark is perceptible at the moment the 

 interruption is effected, the keeper must have this position ; 

 if it appears at a later period, the position of the keeper must 

 correspond with a later stadium of rotation. The difference 

 between the two positions will be greater the quicker the 

 keeper is rotated. Now, when the machine is caused to rotate 

 slowly or quickly in the dark, the keeper, illuminated by the 

 sparks, appears perfectly stationary in that first position, even 

 when viewed by a telescope provided with cross threads directed 

 towards a certain mark upon the keeper. No measurable space 

 of time therefore transpires between the interruption to conduc- 

 tion and the appearance of the spark, although by this means 

 a lesser magnitude than the yo^Jo ^^ ^ second could be mea- 

 sured. 



92. By the researches adduced in this section it has there- 

 fore been proved, that the presence of iron modifies the nega- 

 tive effects of the incipient extra current in the same manner 

 that it modifies the positive effects of the final extra cun-ent, 

 and that both are closely related to the secondary currents as 

 regards all their properties which are susceptible of proof. It is 

 true that these researches only extend to the particular case in 

 which the primary current is a magneto-electric current. But 

 these currents appear at present to offer the only attainable 

 means for instituting such investigations. Besides, without 

 them the fact observed for induction with frictional electricity, 

 that an increased physiological action is effected by bundles of 

 iron wires, and a decreased action by solid iron, would be al- 

 together without analogy. In § 77 of this section, I have suc- 

 ceeded in showing the same phaenomenon by means of Saxton's 

 machine. This appears to indicate, that, as the modifications in 

 the action of iron upon the currents induced by it, according as 

 it is used in the form of solid rods or bundles of iron wires, 

 may be traced to a change in the duration of these currents, so 

 likewise the secondary current of a Leyden jar differs only from 

 the currents induced by other sources of electricity in the iristan- 



