h EXPEKIMENTAL PHYSIOLOGY 



induction shocks, which can of course be obtained by a rapid make and 

 break of the circuit through the primary. To obtain this in an auto- 

 matic way the induction coil is always fitted with an arrangement 

 termed the NEEF'S HAMMER. This is represented in fig. 6, and 

 consists of a pillar d carrying a steel spring to which is attached an 

 iron armature /» . In the centre of this spring is a small platinum 



plate for making contact with 

 the platinum point of a screw 

 adjustable in a brass plate con- 

 nected to the binding screw /, 

 and therefore with one terminal 

 of the primary. Fixed under k 

 is a double electromagnet /, one 

 end of the wire of which is con- 

 nected to h, the second terminal 

 of the primary coil, and the 

 other end to a central pillar /. 

 The mode of action is illustrated 

 by fig. 9. A battery is connected 

 by one pole to the pillar a and 

 by the other to the pillar b, 

 using a mercury key k. If the 

 platinum point of the screw s , be in contact with the platinum plate 

 on the upper surface of the spring v, then on closing the key k the 

 circuit is closed, and if we suppose the positive pole of the battery 

 to be in connection with the pillar a the course of the current is from 

 the battery to a, then along the spring v to the screw s lf thence 

 through the primary coil to the electromagnet, and from this to the 

 second pillar b, and so through the key k back to the battery. 



As soon as the circuit is thus closed the electromagnet acts upon 

 the armature and pulls down the spring v, thereby separating the 

 two platinum surfaces. The current is at once broken, and the electro- 

 magnet therefore ceases to attract the armature, which is carried up 

 by the spring v ; a new contact is thus made by the platinum surfaces, 

 and the whole cycle of events is repeated. In this way the circuit 

 through the primary is made and broken automatically at a rate which 

 depends solely upon the rate of oscillation of the steel spring v. At 

 each make and at each break of the circuit induced currents are pro- 

 duced in the secondary circuit, which, as previously explained, are of 

 very unequal intensities. 



Fig. 9. — To Illustrate the Action 

 of Neef's Hammer. 



Von Helmholtz showed how the Neef's hammer might be arranged 

 to give shocks of about the same intensity. All that is necessary is to- 



