EECENT PROGRESS IN PHYSICS. 



loo 



Fig. 60. 



' circle is joined to the second by the groove c d, (figure fiO,) the second 



' to the third by e /, &c. In these 



! grooves a copper wire about half a line 



'. thick is so laid as to make a spiral. 



' One end of the wire passes throug.h 



I the disk at a, and along the under 



' side to z. From a the wire coils out to 



I c, from c to d, from d to e, /, &c. ; x 



\ y is the other end of the wire thus 



I wound in a flat spiral. 



[ The disk is covered with a thin coat 

 of pitch before placing the wire upon 

 it. 



The wire being fastened by the su- 

 perposition of a hot metallic plate, the 



, spaces between the rin.t!;s of wire will 



I be filled up with the pitch; a heavy heated plate laid on the disk will 



I make the spiral perfectly level. This spiral is now blacked with coal 



I and pressed upon another wooden disk to 

 get the marks for a second spiral, which ^s'^''- 



[ must correspond with the first as nearly as 

 possible 



I The disks are now fastened to glass sup- 

 ports, their planes being vertical. They 



I are arranged upon the same stand opposite 



leach other, and so that they can be ap- 

 proached and separated at pleasure. This 

 arrano;ement is represented in figure 61. 



\ Another arrangement of the flat spiral, 



[much more convenient for many purposes, 

 shown to me by Professor Eisenlohr, of Carlsruhe, is represented in 

 figure 62. One of the spirals is fastened on an upright glass sup- 



Iport in a horizontal position. The second ^.^ ^,, 



1 spiral is fastened in the same manner on a 



[glass rod, which has no foot ; it is placed 



lover the other, like the upper, over the 



(lower condenser plate. 



i The distance between the spirals can be 



I changed by placing glass plates of difler- 



|ent thicknesses between them. For greater 



i distances pieces of varnished wood having 



(any desired thickness are interposed. 



, The ends of the wires are provided with 



i screw clamps z and y, by means of which 



'the spiral can be connected as may be de- 

 sired. 



Placing y and z of the lower spiral from one to two lines apart, and 

 j separating the two spirals by a glass plate, a spark will be seen to 

 , pass between y and z on discharging ajar, sufficiently charged, through 

 ' the upper spiral. The spark is produced by the secondary current. 



