CONDENSER DISCHARGES. 461 



extra current is approximately the same, whether the current in the primary 

 coil is augmented or decreased (Fig. 252, B)-, thus a series of induced 

 currents, recurring at rapid intervals, can be produced, alternate in direction 

 and of approximately equal intensity. Various devices have been employed 

 in order to insure mechanical accuracy in the closing and opening of the 

 contacts effected by the oscillations of the spring or fork ; of these the most 

 effectual is Kronecker's capillary contact, in which the spring causes a platinum 

 wire to dip in and out of mercury, whilst a stream of water removes any 

 electrolytic products formed at the point of contact. A rotator with two 

 strikers and two batteries, coupled up by opposite poles, produces very good 

 results, and obviates the necessity for the introduction of the side wire. 



(2) Undulatory induced currents. For precise stimulation with a 

 rapid series of alternating currents, probably the most effectual method 

 is the employment of a magneto-electric machine. A convenient form 

 consists of two sets of small coils of wire, fixed in a frame a short distance 

 above one another, so as to allow of the rapid passage of a magnet or 

 series of magnets between them. One set of coils is connected with a 

 battery, the other with the exciting circuit, or with the primary coil of 

 an induction machine. The disc with the magnets is rotated by means 

 of a motor at a rapid rate, and a series of precisely similar alternating 

 currents are thus induced in the apparatus. If these are allowed to 

 traverse the primary coil of an induction machine, this acts as a trans- 

 former, and a series of currents are produced in the secondary, the 

 varying intensity of which would, if represented graphically, assume 

 the form of a symmetrically undulating curve (Fig. 252, (7), and may, by 

 appropriate choice of coils, be made strictly sinusoidal in character. 1 



The production of single or slowly recurring currents of similar physical 

 character to those just described, can be obtained by allowing a magnet to fall 

 with constant velocity through the centre of a coil. 2 The elaborate instrument 

 used by Mares for the purpose may be replaced by the following simple form. 



A primary coil containing an internal core of the usual character is fixed 

 upon a pendulum and is connected with a battery ; 3 a secondary coil is 

 fixed immediately below the stationary position of the pendulum and its coil. 

 Each swing of the pendulum produces an effect in the stationary secondary 

 coil, which can be made of such character as to have a current intensity 

 augmenting and diminishing in a perfectly regular way; the effect of several 

 swings is thus to produce a series of alternating currents, each of which is 

 precisely similar as regards its time- relations. By varying the shape of the 

 magnetised core, the time of development may be altered and the variations 

 made sinusoidal in character. The uniform character of the time-relations of 

 these effects insures uniformity in the physical characters of the stimulus. 



(3) Condenser discharges. Theoretically, the condenser discharge is 

 in almost all respects an ideal form of electrical stimulus. 



Its practical employment necessitates several precautions : two 

 condenser plates, on being connected with an electromotive source, are 

 charged up to the PD of this source; on disconnecting, the charge 

 remains ; but on connecting, through a second circuit, including a nerve, 

 the charge rapidly equalises itself, and a current thus flows through 

 the nerve. The rate at which the equalisation occurs depends upon 



1 Burch, Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 231. 



2 Mares, Proc. Internal. Cong., Liege, 1892; Centralbl. f. Physiol. , Leipzig u. Wien, 

 1892, Bd. vi. S. 408. 



3 Gotch and Macdonald, Journ. Physiol. , Cambridge and London, 1896, vol. xx. p. 247. 



