Objects, and on the Oscillating Electric Spark. 257 



were very imperfect, but there is a very slight lateral shift 

 which I believe to be real, but it will be necessary to try the 

 experiment again under more perfect conditions. 



I must express my obligation to our lecture-assistant, Mr. 

 R. Chapman, who has performed the whole of the purely 

 photographic part of the experiments, and whose experience 

 in this direction has been of very great service. 



The following are the particulars of the apparatus that has 

 been used. The electrical constants were determined by 

 Messrs. W. G. Robson and S. W. J. Smith. 



Condenser. 



A wooden tray was made, as shown in tig. 7, 28£ inches 

 long and 16 J inches wide inside, but with the base projecting 

 on each side so as to form a 22-inch square. A sheet of 

 varnished glass of this size was laid upon the base, and then 

 twenty-six pieces of thin window-glass 28 x 16 inches were 

 piled on one another in the tray. The window-glass had 

 been well washed, dried, and varnished while hot on each 

 side all round the edge for a width of two inches. Pieces of 

 tinfoil 24 x 12 inches were laid on each of the oblong- 

 pieces of glass, except the top one, and long tongues of tinfoil 

 3 inches wide were brought out alternately on the two sides. 

 On one side all these tongues were brought out in the middle 

 and bent down so as to rest on the square glass plate below ; 

 on the other side they were brought out in four sets of three 

 each with a space of two inches between each set. These 

 were separately bent down to lie on the square glass plate 

 below. By this means the capacity can be made variable in 

 the proportions of 1, 2, 3, and 4 by simply connecting the wire 

 to 1, 2, 3 or 4 of these electrodes. A strip of tinfoil lying 

 over all the ends and held in contact by weights keeps them 

 all connected except when a smaller capacity is required. 



Total capacity 0*0979 microfarad. 



Total capacity of battery of ten large jars 0*0451 micro- 

 farad. 



Total capacity of small jar used to connect knobs (p. 355) 

 •0026 microfarad. 



Self-induction. 



Two coils, containing 330 yards each of No. 16 copper 

 wire covered with gutta-percha to No. 7, were made use of in 

 the form in which they arrived from the makers. The two 

 coils are marked C and U. The following are the dimensions 

 in inches approximately : — 



