APPENDIX 



429 



The methods adopted have been two, i.e. chemical and 

 mechanical. 



A chemical rectifier can be made by passing the alternating 

 current through a bath with iron and aluminium electrodes ; the 

 bath being filled with a saline solution (phosphate of ammonia is 

 about the best) chokes back the current in one direction and only 

 allows a unidirectional pulsating current to pass. By using four 

 batlis, both phases of the alternating current can be utilised. 



These rectifiers are sold under various names ; the ' Nodon 

 Valve ' is one of the best forms, and can be obtained from any 



Fin. 2-12. Dr. Morton's Rotary Rectifier 



dealer in scientific instruments. I have used such a rectifier a 

 great deal for small currents in charging accumulators, &c., but for 

 heavy currents they are troublesome and wasteful. 



Of mechanical rectifiers, the best and simplest that I have seen 

 was designed by Dr. E. Morton, of the London Hospital (fig. 242). 

 It was originally, and still is, designed chiefly for use in connection 

 with induction coils for #-ray work, but it struck me that the same 

 instrument might be utilised in connection with arc lamps. Dr. 

 Morton very kindly lent me one of these rectifiers for my experi- 

 ments ; and a great deal of good work was done with it. On one 

 occasion I passed a current of 30 amperes for over two hours 

 through this rectifier, and obtained a light that for brightness and 



