290 Prof. Hertz on Rays of Electric Force. 



ductor employed may be most simply described as follows : — 

 Imagine a cylindrical brass body, of 3 centim. diameter and 

 26 centim. length, interrupted in the middle of its length by 

 a spark-space of which the poles on each side consist of sphe- 

 rical surfaces of 2 centim. radius. The length of the conductor 

 is nearly equal to half the wave-length of the corresponding 

 oscillations in straight wires ; we can thus form at once an 

 approximate estimate of the period of oscillation. It is essen- 

 tial that the pole-surfaces of the spark-interval should be 

 frequently polished, and during the experiments carefully 

 guarded against illumination by simultaneous lateral dis- 

 charges, otherwise the oscillations are not obtained. 



The aspect and the sound of the spark both give notice 

 whether the spark-interval is in a satisfactory condition. The 

 discharge is conducted to the two halves of the conductor by 

 means of two thick wires covered with gutta-percha, which 

 terminate near the spark-interval on the two sides. As induc- 

 torium I found it advantageous to employ, instead of the 

 large Iluhmkorff''s apparatus, a small coil by Keiser and 

 Schmidt, capable of giving sparks of, at most, 4'5 centim. 

 length. It was worked by three accumulate rs^ and gave 

 between the spherical surfaces of the primary conductor 

 sparks of from 1 to 2 centim. long. The spark- interval was 

 then adjusted for the experiments to a length of about 

 3 millim. 



The esddence of the electric forces in space was obtained 

 here also by means of the small sparks which they produced in 

 a secondary conductor. As before, I made use partly of a 

 movable circle, which had an oscillation-period nearly equal 

 to that of the primary conductor. This had now a diameter 

 of 7*5 centim., and was constructed of copper wire 1 millim. 

 thick. The one end of the wire carried a polished brass ball 

 of some millimetres diameter ; the other end was pointed, and 

 was adjusted by means of a fine screw, insulated from the 

 wire, to an extremely small distance from the brass ball. It 

 will easily be understood that we have to do only with little 

 sparks of a few hundredths of a millimetre in length, and a 

 little practice enables one to judge better from the brightness 

 of the sparks than from their length. 



The circular conductor gives only a diflferential effect, and 

 is not suitable for placing in the focal line of a concave 

 mirror; it was therefore combined with another secondary 

 conductor of the following form : — Two straight pieces of 

 wire, 50 centim. long and 5 millim. diameter, were so ar- 

 ranged in the same straight line that the opposing ends were 

 5 centim. apart. From these ends two wires, 15 centim. 



