266 The Rev. Prof. Callan on a New Single 



exciting fluid, and when the battery is intended not for constant 

 work, but for experiments which may be occasionally interrupted, 

 to make provision for taking tbe metallic plates out of the exci- 

 ting fluid whenever it may be necessary to suspend the expe- 

 riments. 



First, the greater the proximity of the metallic elements, the 

 more powerful will be the galvanic action, provided the hydrogen 

 escapes with sufficient freedom to allow the exciting fluid to be 

 in continual contact with the metals. If the hydrogen prevents 

 the continual contact of the exciting fluid and metals, there will 

 be a series of voltaic currents instead of one constant uniform 

 current. I have found that when the zinc plate is not more than 

 4 inches square, the hydrogen does not interfere with the constant 

 action of the battery, even though the distance between the zinc 

 and cast iron should not exceed the ^th of an inch. Even in 

 that case a perfectly steady deflection of the magnetic needle is 

 produced by the battery, and therefore the voltaic current must 

 be constant and steady. I have not yet tried plates larger than 

 4 inches square, but I am inclined to think that plates a foot 

 square or more need not be more than y^-th of an inch asunder. 



Secondly, the part of the cast iron which has little or no effect 

 in producing the voltaic current, must be protected against the 

 action of the exciting fluid. Each of the exciting fluids that 

 have been described acts a good deal on cast iron ; and were the 

 inactive part of the iron unprotected, there would be a consider- 

 able and useless consumption of cast iron, and of the strength 

 of the exciting fluid. The iron may be sufficiently protected by 

 covering it with vulcanized india-rubber, wood, or any other sub- 

 stance on which the exciting fluid acts but little or not at all. 



The cast iron may have the form of cells which hold the ex- 

 citing fluid, or of plates, each pair being connected together at 

 the top, but separated from each other elsewhere so as to admit a 

 zinc plate between them. The distance between each cast-iron 

 plate and the one connected with it at the top may be a quarter 

 or a fifth of an inch, so that a zinc plate an eighth of an inch 

 thick may be put between the two, and that each side of the 

 zinc will be a sixteenth or twentieth of an inch from the cast 

 iron. The contact between the zinc and cast iron may be pre- 

 vented by a thin wedge of wood at each corner of the plate. If 

 cast-iron cells be used, they must be an inch or an inch and a half 

 higher than the zinc plates. The width of the part of the cell which 

 contains the zinc plate should not exceed one-fourth of an inch, 

 otherwise the distance between the zinc and cast iron will be too 

 great, and there will be a loss of galvanic power. The part of 

 the cast-iron cell which is near the top or above the zinc plate 

 should be about 1£ inch wide, in order to hold a sufficient 



