86 



Lord Kelvin on 



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), demonstrating 

 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 connexion 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 were 

 slightly tilted in the lifting, so as to break the water arc 

 before the separation between the metals, and secure that the 

 last connexion is contact of dry metals. To show this to you 

 more readily than by a Volta condenser with gold-leaf elec- 

 troscope, I shall now use instead my quadrant electrometer 

 without 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 electro- 

 meter. 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 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 



bring 



down the zinc disc 



to touch the top of the little mound of water, keeping it 

 parallel to the copper disc so that there is no metallic contact 



