346 Sir William Thomson [April 10, 



The doctrine of electric images is slightly alluded to, and an 

 illustrative experiment performed, showing the fixing of an electric 

 image. The electroscope used for the experiments is an electrified 

 pith ball, suspended by a varnished double-silk fibre of about 9 or 

 10 feet long. Figs. 1-4 represent experimental illustrations, in 

 which the pith ball, positively electrified, experiences a force due to 

 electrified bodies, optically screened from it by a thin sheet of tin- 

 plate. In Figs. 1 and 2 the pith ball is attracted round a corner by 

 a stick of rubbed sealing-wax, and in Figs. 3 and 4 repelled round a 

 corner by a stick of rubbed glass. In Fig. 2 the sealing-wax seems 

 to repel the pith ball, and in Fig. 4 rubbed glass seems to attract it. 



Fig. 1. 



-® 



Fig. 2. 



Fig. 3. 



©--• 



Fig. 4. 



This experiment constituted a very palpable illustration of Faraday's 

 induction in curved lines of force. 



In the present lecture some experimental illustrations were given 

 of electrostatic screening by incomplete plane sheets and curved 

 surfaces of continuous metal, and of imperfectly conducting material, 

 such as paper, slate, wood, and a sheet of vulcanite, moist or dry, 

 window glass at ordinary temperatures in air of ordinary moisture, 

 and by perforated metal screens and screens of network, or gratings 

 of parallel bars. 



