7 i2 TEXT-BOOK OF PHYSIOLOGY. 



Inside the primary coil there is placed a bundle of soft iron wires, which, 

 as soon as the circuit is made, become magnetized, with the effect of in- 

 creasing the action of the inducing current. 



The secondary coil, R", consists of a much greater number of turns of 

 a finer copper wire, the ratio being about 40 to i, also wound around a spool, 

 having a tunnel sufficiently large to enable it to slide over the primary. By 

 these means the strength of the induced current is increased . As a result of the 

 construction of the inductorium, the low electro-motive force of the cell is trans- 

 formed into the high electromotive force characteristic of the induce d current. 

 As the number of turns of wire in the secondary coil is to the number in the 

 primary, so are the electro-motive forces in the secondary coil to those in the 

 primary coil. 



The secondary coil slides along a track, B, which permits it to be moved 

 toward or away from the primary. The distance between the two coils can 

 be measured and the strength of the induced current again reproduced, 

 other things being equal, by means of a centimeter-millimeter scale pasted on 

 the edge of B. 



The ends of the wire of the secondary coil are fastened to two binding 

 posts to which conducting wires provided with hand electrodes can be 

 attached. 



The inductorium may be used for obtaining either a single current or a 

 series of rapidly repeated induced currents. 



The Single Induced Current. On account of its high electro-motive force, 

 its penetrative power, and short duration, the single induced current is a 

 most convenient and suitable form of stimulus for many purposes. In 

 order to obtain such a current, the positive wire of the cell is carried to 

 binding post S", and the negative wire either to S'" or P". A key is placed in 

 the primary circuit. The course of the current will then be on the closure of 

 the circuit from the cell to S", thence around R' to S'", and so back to 

 the cell; or if the negative wire is connected with P", the course of the cur- 

 rent on leaving R' will be through the coils surrounding the two vertical bars 

 B', thence to P", and so back to the cell. It the secondary coil be placed 

 close to the primary and the wires of the secondary brought into contact 

 with a muscle, it will be found that with both the make and the break of 

 the primary circuit a current is induced in the secondary, as shown by a 

 short quick pulsation of the muscle; but during the time of closure of the cir- 

 cuit the induced current is wanting, as shown by the quiescent condition of the 

 muscle. It will be apparent, however, from the energy of the contraction 

 that the break-induced current is a more efficient stimulus than the make- 

 induced current. That this is the case is made evident by removing the 

 secondary to the end of the slide- way and then gradually bringing it toward the 

 primary half a centimeter at a time, making and breaking the circuit after each 

 movement until a pulsation of the muscle occurs. It will be found to occur 

 first on the break of the circuit. As the secondary approaches the primary a 

 position will be reached when a pulsation occurs on the make as well as on 

 the break of the circuit, though it will be less pronounced. 



The explanation offered for this difference in the strength of the two induced currents is as 

 follows: With the make of the circuit and the passage of the battery current through the primary 

 coil there is induced in the neighboring and parallel turns of the wire and extra current opposite in 



