148 



hertz's researches on electrical waves. 



tbe greater its length the greater will be the retardation of the electrical 

 wave in its passage through it from one knob of the micrometer to the 

 other. 



The material, the resistance, and the diameter, of the wire of which 

 the micrometer circuit is formed, have very little influence on the spark. 

 The potential variations can not therefore be due to the resistance, and 

 this was to be expected, for the rate of propagation of an electrical dis- 

 turb;) nee along a conductor depends mainly on its capacity and co-effi- 

 cient of self-induction, and only to a very small extent on its resistance. 

 The length of the wire connecting the micrometer circuit with the 

 secondary circuit of the coil is also found to have very little influence, 

 provided it does not exceed a few meters in length. The electrical dis- 

 turbances must therefore traverse it without undergoing any appreciable 

 change. Tbe position of the point of the micrometer circuit which is 

 joined to the secondary circuit, is on the other hand of the greatest 

 imj)ortance, as would be expected, for if the point is placed symmetric- 

 ally with respect to the two micrometer knobs the variations of poten- 

 tial will reach the latter in the same phase, and there will bo no eff"ect, 

 as is verified by observation. If the two branches ot the micrometer 

 circuit on each side of the point of contact of the connection with the 

 secondary are not symmetrical, the spark can not be made to disappear 

 entirely ; but a minimum efiect is obtained when the point of contact is 

 about half-way between the micrometer knobs. This point may be 

 called the null i)oint. 



Fig. 2 sliows f lio arrangement employed, c being the null point of the 

 rectangular circuit, which is 125 centimeters long 

 l>y SO centimeters broad. When the point of con- 

 tact is at a or />, sparks of from 3 to 4 millimeters 

 in length are observed, when it is at e no sparks 

 are seen, but they can be made to re-appear by 

 ■shifting the point of contact a few centimeters to 

 the right or left of the null point. It should be 

 noted that sparks only a few hundredths of a milli- 

 lueter in length can be observed. If when the 

 point of contact is at e another conductor is placed 

 in contact with one of the micrometer knobs the 

 sparks re-appear. 



Now the addition of this conductor can not pro- 

 duce any alteration in the time taken by the dis- 

 turbances proceeding from e to reach the knobs, 

 and therefore the phenomenon can not be due simply to single waves 

 in the directions c a and d h respectively, but must be due to repeated re- 

 flection of the waves until a condition of stationary vibration is attained, 

 and the addition of the conductor to one of the knobs must diminish or 

 prevent the reflection of the waves from that terminal. It must be as- 

 sumed then, that definite oscillations are set up in the micrometer cir- 



1 



Fig. 2. 



