652 



Professor J. A. Fleming 



[June 4, 



I 



at any one station both for sending and receiving, and to switch it 

 over from the transmitter to the receiver according as we wish to send 

 or receive messages, although methods have been described and are 

 being developed for usingthe antenna simultaneously for both purposes. 

 By way of preface let me illustrate by a few experiments the 

 manner in which these electric oscillations are set up in the air-wire, 

 and the nature of the effects produced by them in the surrounding 

 space. We have here a very long wire wliich, for the purpose of 

 keeping it within a small compass, is coiled upon an ebonite tube. 

 Two such spirals, H^ and H.,, are placed side by side and connected at the 

 bottom through two other small coils of wire S (see Fig. 1). In con- 

 tiguity to these last two coils of wire are two others, P, which are in series 

 with a condenser or battery of Leyden jars, C, and a spark gap. If we 

 charge the condenser by an induction coil, I, and let it discharge across 

 the gap, we produce rapidly succeeding trains of electric oscillations in 



the condenser circuit, and these induce 

 other currents in the open or helix 

 circuit of similar kind. The result is 

 that electricity rushes up and down the 

 spiral wires, which we may consider to 

 represent two very long air wires or 

 antennae. We have therefore alternately 

 free charges of electricity at the top 

 ends of the wires and electric currents 

 passing to and fro across the middle 

 point. We may compare this movement 

 of electricity in the helix to the oscilla- 

 tions of a liquid in a U-tube when it 

 is disturbed. In the electrical case we 

 have at each spark- discharge 20 or 30 

 electrical swings or oscillations, sepa- 

 rated by relatively long intervals of silence, the intervals between two 

 swings in the train being about one four-hundred-thousandth of a 

 second, whilst the interval between the groups or trains of swings 

 is about one-fiftieth of a second. 



Such electrical oscillations in the wire produce two effects in 

 external space, called respectively electric and magnetic force. In 

 the case of a simple vertical air-wire, the magnetic force is distributed 

 along concentric circular lines embracing the Avire, whilst the electric 

 force is distributed along certain looped lines in the plane of the wire. 

 If, however, we employ a close-wound spiral antenna as in our 

 experiment, the positions of the electric and magnetic forces are 

 interchanged as compared with those of the single vertical wire. 



As the currents in the air-wire reverse their direction, the magnetic 

 and electric effects in the external space also reverse, but not every- 

 where at the same moment. The magnetic and electric forces are 

 affections or states of the tether, and in virtue of the inertia and 



^ZjlZ0 



T 



Fig. 1. 



