HERTZ S RESEARCHES ON ELECTRICAL WAVES. 



155 



When the wire c <l was suiTouiuled by an iron tube, or when it was 

 replaced by an iron wire, no perceptible effect was obtained, confirm- 

 ing- the conclusion previously arrived at that the magnetism of the iron 

 is unable to follow such rapid oscillations, and therefore exerts no ap- 

 preciable eflect. 



Nodes. — The vibrations in the micrometer circuit which have been con- 

 sidered are the simplest ones possible, but not the only ones. While 

 the potential at the ends alternates between two fixed limits, that at the 

 central portion of the circuit retains a constant mean value. The elec- 

 trical vibration therefore has a node at the center, and this will be the 

 only nodal point. Its existence may be proved by placing a small insu- 

 lated sphere close to various portions of the micrometer circuit while 

 sparks are passing at the discharger of the coil, when it will be found 

 that if the sphere is placed close to the center of the circuit the spark- 

 ing will be very slight, increasing as the sphere is moved farther away. 

 The sparking cannot however be entirely got rid of, and there is a bet- 

 ter way of determining the existence and position of the node. After 

 adjusting the two circuits to unison, and drawing the micrometer termi- 

 nals so far apart that sparks can only be made to pass by means of 

 resonant action, let different parts of the circuit be touched by a con- 

 ductor of some capacity, when it will be found that the sparks disap- 

 pear, owing to interference with the resonant action, except when the 

 point of contact is at the center of the circuit. The author then en- 

 deavored to produce a vibration with two nodes, and for this purpose 



Fic. 7. 

 he modified the apparatus previously used by adding to the micrometer 

 circuit a second rectangle e/// li exactly similar to the first (as shown 

 in Fig. 7), and joining the points of the circuit near the terminals by 

 wires K» and 24, as shown in the diagram. 



