EIeclro-Mag7ietic Experiments. 45 



doubled, without the effect being greatly diminished. When tiie 

 surface of the two metals is small, the effect is proportionally di- 

 minished ; and vice versa, it is increased in proportion as the 

 surfaces are increased. A considerable effect is obtained from a 

 zinc plate six inches square, immersed in a copper vessed filled 

 with the liquid conductor which I have just mentioned. An ap- 

 paratus of this description, in which the surface of the zinc plate 

 is two inches square, acts upon the needle with so much force, 

 that the effect is sensibly felt at the distance of three feet, and 

 that too, when the needle is not very moveable. I have not met 

 with any greater effects from an apparatus composed of forty si- 

 milar troughs ; indeed, the effect appeared somewhat diminished. 

 I have not pursued the investigation of this point very minutely, 

 but the observation which I have made is correct. I shall con- 

 clude that by the slight diminution of the conducting power, 

 which results from an increase in the number of the elements of 

 the apparatus, a diminution in tlie electro-chemical effect is also 

 occasioned. 



In order that the effect of a single galvanic arc may be com- 

 pared with that of an apparatus consisting of several arcs or ele- 

 ments, the following experiment mav be made. Suppose fig. 4. 

 (Plate I.) to represent a galvanic arc, consisting of a piece of 

 zinc Zy of copper e, of a metallic wire a h, and of a fluid conduc- 

 tor /. The zinc invariably communicates to the water a portion 

 of its positive electricity, and the copper a portion of its negative. 

 Tn consequence of this, there would be an accumulation in the 

 upper part of the zinc of negative electricity, and in the upper part 

 of the copper of positive electricity, if it were not that the commu- 

 nication a I) established the equilibrium, by furnishing a free pas- 

 sage for the negative electricity from c to z. It will be seen, 

 then, that the wire a b receives the negative electricity of the zinc, 

 and the positive electricity of the copper ; while a wire, which 

 forms the communication of the two poles of a |)ile, or of another 

 compound galvanic apparatus, receives the positive electricity of 

 the zinc pole, and the negative of the copper pole. 



Paying proper attention to this distinction, all the experiments 

 which I at iirst made with a compound galvanic apparatus, may 

 be repeated with a single galvanic arc. The use of a single gal- 

 vanic arc is attended with this great advantage, tliat it enal)les 

 the experiments to be repeated with little expense and trouble. 

 It has another advantage still more considerable, namely, that a 

 galvanic arc may be formed of power suflicient for the electro- 

 magnetic experiments, and yet light enough to besosiispended to 

 a small metallic wire, as that the small apparatus may be turned 

 round the prolonged axis of tiic wire. It is open, in this way, to 



examine 



