54 PHYSIOLOGICAL PHYSICS. [Chap. vi. 



magnetism to remain permanently. The discovery of 

 the electro-magnet is due to Arago. How it is adapted 

 for the automatic interruption of a current has been, 

 explained in noticing the construction of the induction 

 coil (page 39). The electro-magnet is also very ex- 

 tensively employed in the making of electromotors, 

 telegraph instruments, etc. 



The luminous effects of the electric current 

 it is not necessary to discuss here. A brief reference 

 will be found in the chapter on the application of 

 electricity to medicine and surgery (chap. xv.). 



The effects of a current 011 a magnet are 

 discussed in the chapter on galvanometers (chap. x.). 



The physiological effects of the electric current 

 form the subject of other chapters. 



CHAPTER VI. 



KEYS, COMMUTATORS, AND ELECTRODES FOR PHYSIO- 

 LOGICAL PURPOSES. 



A GALVANIC key is for the purpose of rapidly and 

 easily making or breaking a galvanic circuit. This 

 could be done, of course, by simply loosening one of 

 the wires from its connections with the battery, or 

 with the apparatus to which the current is conveyed ; 

 but the use of a key affords a great convenience. 

 There are various forms in general use. 



A mercury key is shown in Fig. 29. It con- 

 sists of a circular block of wood which can be clamped 

 to the table. In. the centre is sunk a small porcelain 

 cup which contains clean mercury. A thick copper 

 wire is fixed across the block, with its end curving 

 down into the mercury. The projecting end is much 

 thicker than the rest, and is hollowed out so that a 



