HISTORY, &c., OF INDUCTION INSTRUMENTS. 321 



Fig. 92. — Stiihrer's larger induction apparatus. 



atus. The battery of the former is constituted by a single cell ; of the latter 

 by two cells, which may, however, be arranged either as two pairs, or as a 

 single pair of elements. Both possess a primary and a secondary coil, the 

 currents of each of which can be made use of separately; and, in both, 

 the currents have a definite direction, positive electricity being set free at one 

 terminal, and negative at the other, of each of the coils. The terminal from 

 which the positive current proceeds may be ascertained easily by the decom- 

 position of iodide of potassium. The larger apparatus differs from the smaller, 

 in having a much greater range of power, more thorough means of graduating 

 the currents, and a more elaborate arrangement of the interrupting hammer. 

 With practice, however, the force and rate of interruption of the smaller 

 apparatus may be regulated with much nicety. To neither instrument is a 

 water-graduator attached ; but, if needed for any special nicety of application, 

 one can readily be added, and it would be best carried loose in the drawer for 

 accessories, to be attached only when required. Graduation of the strength 

 of the currents is effected by the degree of immersion of the voltaic elements 

 in the exciting acid, and by the arrangement of the coils. The primary coil 

 is fixed vipon a pedestal ; the secondary is movable, and is brought into or 

 placed out of action by being lifted over or thrust away from the primary coil. 

 The degree of action in the secondary coil is iiroportionate to the extent to 

 which it is brought under the influence of the primary coil. The action of the 

 primary coil is regulated in the smaller instrument by the degree of immersion 

 of the voltaic elements, and by the extent to which the coil is masked by the 

 secondary coil, — the latter coil acting upon it, when the primary coil is alone in 

 use, as a metallic sheath would do. In the larger apparatus, a special copj^er 

 sheath is provided for the graduation of the current of the primary coil. It 

 is to be regretted that a similar arrangement has not been adopted in the 

 smaller apparatus. 



