CHAP, iv.] ELECTRICITY AND MAGNETISM. 229 



compressed the glass and caused it to expand laterally. 

 This idea had previously occurred to Priestley. Duter 

 showed that the amount of apparent expansion was 

 inversely proportional to the thickness of the glass, and 

 varied as the square of the potential difference. Quincke 

 has recently shown that though glass and some other 

 insulators exhibit electrical expansion, an apparent con- 

 traction is shown by resins and oily bodies under 

 electrostatic stress. He connects with these properties 

 the production of optical strain and of double refraction 

 discovered by Kerr. (See Lesson on Electro-optics, 

 Art. 386.) 



274. Submarine Cables as Condensers. A 

 submarine telegraph cable may act as a condenser, the 

 ocean forming the outer coating, the internal wire the 

 inner coating, while the insulating layers of guttapercha 

 correspond to the glass of the Leyden jar. When one 

 end of a submerged cable is connected to, say, the + pole 

 of a powerful battery, + electricity flows into it. Before 

 any signal can be received at the other end, enough 

 electricity must flow in to charge the cable to a consider- 

 able potential, an operation which may in the case of 

 long cables require some seconds. Faraday predicted 

 that this retardation would occur. It is, in actual fact, a 

 serious obstacle to signalling with speed through the 

 Atlantic cables and others. Professor Fleeming Jenkin 

 has given the following experimental demonstration of 

 the matter. Let a mile of insulated cable wire be coiled 

 up in a tub of water (Fig. 105), one end, N, being 

 insulated. The other end is joined up through a long- 

 coil galvanometer, G, to the + pole of a large battery, 

 whose pole is joined by a wire to the water in the tub. 

 Directly this is done, the needle of the galvanometer will 

 show a violent deflection, + electricity rushing through it 

 into the interior of the cable, and a - charge being 

 accumulated on the outside of it where the water touches 

 the guttapercha. For perhaps an hour the flow will go 



