348 



RECENT PROGRESS IN PHYSICS. 



Each of these figures is composed of the two elementary forms 

 wliich Lichtenberg has distinguished as positive and negative, and 

 for this reason the direction of the secondary current cannot be de- 

 duced I'rom these figures. 



In the ibllowing experiments the ends of the secondary spiral were 

 lengthened by copper wires, and a part of one formed a short, close 

 coil, wound to the right. In fig. 67 let x and y indicate the ends of 

 the secondary spiral to which the above-mentioned wires are attached. 1 

 To magnetize a steel needle the ends a and /? were 

 ^'"■^'^* put in contact, and the needle was placed in the coil, 1 



with its point toward m. To obtain the figures on j 

 the resin it was introduced between a and ^. The i 

 results contained in the following table were obtained i 

 with the small main and secondary spirals, consisting { 

 of 13 feet of copper wire, already mentioned. 



In the main spiral the discharge current passed in i 

 the direction indicated by the arrow. The ibllowing » 

 table shows the polarity indicated by the needle when c 

 it lay in the coil pointing towards m. 

 A glass plate was interposed between the two spirals. 



ffk OI-jP 



It is seen that for the same direction of the main current the mag- 

 netism of the needle varies with the other circumstances, whence a 

 difference in the directions of the second-ary current might be deduced; 

 but the resin plate being interposed between a and /9, and the battery 

 discharged through the main spiral under all the circumstances given 

 in the table, fig. 65 was constantly formed on the side of the resin 

 plate turned toward the end of the wire /? — a proof that the direction 

 of the secondary current remained the same^ though the magnetism 

 of the needle was reversed. 



Eiess used for producing the figure a small glass or copper plate, 

 both sides having been covered with a thin coating of pitch or resin. 



A surface of resin once used must be heated over the flame of a 

 spirit lamp to melting before it can be employed again. 



The direction of the secondary current, which, as already remarked, 

 could not be directly determined from the figures of the resin plate, 

 was ascertained in the following way : Two three-inch condensers 

 were separated by a thin plate of mica ; the lower one touched the 



