342 Lord Kelvin on a Piezo-electric Pile. 



dimension parallele de l'axe optique n'a pas d ''influence sur 

 la quantite d'electricite degagee *. 



La lame de quartz est placee dans une enceinte metallique 

 dessechee. Cette cage metallique, toutes les pieces metalliques 

 de ^instrument et les montures de la lame de quartz sont 

 mises en communication permanente avec la terre. 



Le modele no. 2 comporte encore un commutateur et un 

 levier qui sert a soulever les plateaux et les poids. Nous 

 reviendrons plus loin sur le role de ces organes. 



XXXIV. On a Piezo-electric Pile. By Lord Kelvin f. 



ri^HE application of pressure to a Voltaic pile, dry or wet, 

 J- has been suggested as an illustration of the piezo-electric 

 properties of crystals, but no very satisfactory results have 

 hitherto been obtained, whether by experiment or by theo- 

 retical considerations, so far as I know. Whatever effects of 

 pressure have been observed have depended upon complex 

 actions on the moist, or semi-moist, substances between the 

 metals, and electrolytic or semi- electrolytic and semi-metallic 

 conductances of these substances. Clearing away everything 

 but air from between the opposed metallic surfaces of different 

 quality, I have made the piezo-electric pile which accompanies 

 this communication. It consists of twenty-four double plates, 

 each 8 centimetres square,, of zinc and copper soldered together, 

 zinc on one side and copper on the other. Half a square 

 centimetre is cut from each corner of each zinc plate, so that the 

 copper square is left uncovered by the zinc at each of its four 

 corners. Thus each plate presents on one side an uninterrupted 

 copper surface, and on the other side a zinc surface, except 

 the four uncovered half square centimetres of copper. A 

 pile of these plates is made, resting one over the other on 

 four small pieces of india-rubber at the four copper corners. 

 The air-space between the opposed zinc and copper surfaces 

 may be of any thickness from half a millimetre to 3 or 4 

 millimetres. Care must be taken that there are no minute 

 shreds of fibre or dust bridging the air-space. In this respect 

 so small an air-space as half a millimetre gives trouble, but 

 with 3 or 4 millimetres no trouble is found. 



The lowest and uppermost plates are connected by fine 



* Double breadth, with doubled stretching force, would give double 

 quantity. 



t Communicated by the Author, having been read before Section A 

 of the British Association at its recent meeting in Nottingham. 



