254 Electricity and Galvanism. 



than the well-known Cruikshank trough, which consists of a wooden 

 trough, having double plates of copper and zinc fixed in at short 

 intervals from each other. These plates need not be more than two 

 inches square, and a trough containing three or four dozen pairs will 

 be sufficient for all purposes. The best exciting fluid is very dilute 

 hydrochloric acid, made by mixing one part of the acid with thirty 

 of water. "When the acidulated fluid is poured into the trough, you 

 must take care that it does not rise to the top of the plates by about 

 one quarter of an inch. In using this apparatus, a piece of copper 

 wire should be twisted at one end into a loose coil, and plunged in 

 the first cell of the battery, another similar piece being immersed 

 into the last cell. These wires become the conductors, or electrodes, 

 or, in other words, their free ends represent doors, out of which 

 currents of the two electricities escape ; and, by placing them in 

 contact with the surface of the body, previously moistened, to make 

 it as good a conductor as possible, the union of the two electric 

 elements will take place in the tissues they traverse. Bearing in 

 mind the facts I announced to you in connection with the course 

 traversed by currents, with the development of certain phenomena 

 of nervous irritability and muscular contraction, you will at once 

 see the importance of being able in an instant to ascertain the direc- 

 tion of the two currents when excited by the action of the acid on 

 the zinc and copper plates. This you can at once discover by 

 looking at the trough, and remarking that the positive current 

 escapes from the end towards which all the zinc plates look, and the 

 negative current from the other end. 



The great drawback to the utility of this mode of exciting electri- 

 city is the trouble of getting the apparatus in proper order, the 

 irregularity of the current in regard of strength, its tension and 

 quantity rapidly sinking from the first moment of adding the acid : 

 and, lastly, the damage inflicted by the latter when ejected from the 

 trough from too violent an effervescence, or from its being acci- 

 dentally spilled. 



The next mode of exciting electricity is of late discovery, — one of 

 the many contributions to physical science for which we are indebted 

 to the talents of our illustrious countryman, Dr. Faraday. It fur- 



