78 Professor G. F. Fitz Gerald [March 21, 



such a thing were attainable, a magnet alternating in polarity, might 

 be called a magnetic oscillator. A ring magnet with a closed 

 magnetic circuit is essentially an electric oscillator, while a ring of 

 ring magnets would be essentially a magnetic oscillator again. The 

 elementary theory of a magnetic oscillator can be derived from that 

 of an electric oscillator by simply interchanging electric and magnetic 

 force. Electricity and magnetism would be essentially interchange- 

 able if such a thing existed as magnetic conduction. The only 

 magnetic currents we know are magnetic displacement currents and 

 convection currents, such as are used in unipolar and some other 

 dynamos. It is in this difference that we must look for the difference 

 between electricity and magnetism. * 



In order to observe the existence of these electromagnetic oscilla- 

 tions we can employ the principle of resonance to generate oscillations 

 in a system whose free period of oscillation is the same. A magnetic 

 receiver may be employed consisting of a single incomplete circle of 

 wire broken by a very minute spark-gap, across which a spark leaps 

 when the oscillations in the wire become sufficiently intense. In 

 order that a large audience may observe the occurrence of sparks, 

 the terminals of a galvanometer circuit were connected one with one 

 side of the sjjark-gaj) and the other with a fine point which could be 

 approached very close to the other side of the spark-gap. It was 

 observed that when a spark occurred in the gap, a spark could also 

 be arranged to occur into the galvanometer circuit, and with a delicate 

 long-coil galvanometer (that used had 40,000 ohms resistance) a very 

 marked deflection can be produced whenever a spark occurs. This 

 arrangement we have only succeeded in working comparatively close 

 to the generator, because the delicacy required in adjusting the two 

 spark-gaps is so great. It can, however, be employed to show that 

 the sparks produced in this magnetic resonant circuit are due to 

 resonance by removing this receiver from the generator to such a 

 distance that sparks only just occur, and then substituting for the 

 single circuit a double circuit, which, except for resonance, should 

 have a greater action than the single one, but which stops the sparking 

 altocrether. An electric receiver was also used which was identical 

 with the generator, and had a corresponding, only much smaller, 

 spark-gap between the two plates. When the plates are connected 

 with the terminals of the galvanometer, upon the occurrence of each 

 spark the galvanometer is deflected. It is not so easy to obtain 

 sparks when the plates are connected with the galvanometer as when 

 they are insulated, and it is this that has limited the use of this 

 method of observation. By making the first metre or so of the wires 

 to the galvanometer of extremely fine wire, so as to reduce their 

 capacity, we have found tliat the difficulty of getting sparks is less 

 than with thick wires. We have not observed any effect due to the 

 thickness of the wires after a short distance from the receiver. 



In the case of the small oscillator, a receiver exactly like the one 

 described by Prof. Hertz in his second paper already quoted was 



