624 



Lord Kelvin 



[May 21, 



to the Britisli Assocation of that year, ' On the Seat of the Electro- 

 motive Forces in the Voltaic Cell.' 



§ 5. The reason for this unmerited neglect of a great discovery 

 regarding properties of matter is that it was overshadowed by an 

 earlier and greater discovery of its author, by which he was led to 

 the invention of the voltaic pile and crown of cups, or voltaic battery, 

 or, as it is sometimes called, the galvanic battery. Knowing, as we 

 now know, both Volta's discoveries, we may describe the earlier 

 most shortly by saying that the simple experiment (§ 1 above), de- 

 monstrating the later discovery, is liable to fail if a drop of water 

 is placed on the lower of the two polished plates. It fails if (see 

 Fig. 4 below) the last connection between the zinc and copper, when 

 the upper disc is lifted, is by water. It would not fail (see Fig. 6 below) 

 nor be sensibly altered from what is found with the dry polished 

 metals, if the upper disc is slightly tilted in the lifting, so as to 

 break the water arc before the separation between the metals, and 

 secure that the last connection is contact of dry metals. To show 

 this to you more readily than by a Volta condenser with gold leaf 

 electroscope, I shall now use instead my quadrant electrometer with- 

 out condenser. 



(1) Holding the copper disc connected with the metal case of 

 the electrometer in one hand, with my other hand I hold by a glass 

 handle the zinc disc, which you see is connected by a fine wire with 

 the insulated quadrants of the electrometer. I first place the zinc 

 resting on the copper, both being polished and dry. You now see 

 the spot of light at the point marked O on the scale, which I call 

 the metallic zero. I now lift the zinc disc two or three millimetres 

 from resting on the copper, and you see the spot of light travelling 

 largely to the right, which proves that vitreous electricity has passed 

 from the zinc disc through the connecting wire to the insulated 

 quadrants of the electrometer. I lower the zinc disc down to rest 

 again on the copper disc ; you see the spot of light again comes back 

 to the metallic zero. 



(2) I now raise the zinc disc, and with a little piece of wet wood 

 (or a quill pen) place a little mound of water on the copper disc, as 

 shown in Fig. 3. I bring down the zinc disc to touch the top of the 



Fig. 3. 



