672 Professor J. A. Fleming [June 4, 



signals. But another station not so tuned will either receive nothing 

 at all or else a continuous unl)roken line or sound not having any 

 meaning. There are other methods by which signals not intended 

 for a particular receiver can be rejected by it. Fessenden has 

 described for this purpose an interference detector, in which the 

 impulses it is not desired to receive are made to divide between two 

 paths, the oscillations in which are then caused to neutralise each 

 other's effect on the oscillation detector. On the other hand, the 

 waves of the wave-length it is desired to receive do not so neutralise 

 themselves, but produce a signal by their operation on the detector. 



We must pass on to notice in the next place some improvements 

 in oscillation detectors, and means of testing them. As already ex- 

 plained, the aether waves sent out by the transmitting antenna fall on 

 the receiving antenna and create in it or some other circuit connected 

 to it very feeble oscillations. These oscillations being very feeble 

 alternating currents of high frequency, cannot directly affect either 

 an ordinary telegraphic instrument or a telephone, but we have to 

 interpose a device of some kind called an oscillation detector, which 

 is affected by oscillations in such a manner that it undergoes some 

 change which in turn enables it to create, increase, or diminish a local 

 current produced by a local battery and so affect a telephone or tele- 

 graphic relay. One kind of change the oscillations can produce in 

 certain devices is a change in their electric resistance, which in turn 

 is caused to increase or diminish a current through a telephone or 

 telegraphic relay generated by a local battery. To this type belong the 

 well known coherers of Branly, Lodge and Marconi, which require tap- 

 ping or rotating to bring them back continually to a condition of sen- 

 sitiveness. Coherers, however, have been devised which require no 

 tapping. Thus it has been found by Mr. L. H. Walter, that if a short 

 length of very fine tantalum wire is dipped into mercury there is a very 

 imperfect contact between the mercury and tantalum for low electro- 

 motive forces. This may perhaps arise from the fact that tantalum, 

 like iron, is not wetted by mercury. If, however, feeble electric os- 

 cillations act between the mercury and tantalum, the contact is im- 

 proved whilst they last. If, then, the terminals of a circuit containing 

 a telephone in series with a shunted voltaic cell are connected to the 

 mercury and tantalum respectively, and if damped or intermittent 

 trains of electric waves fall on an antenna and excite oscillations which 

 are allowed to act on the mercury tantalum junction, then at each 

 train the resistance of the contact falls, the local cell sends current 

 through the telephone and produces a short sound, and if the trains 

 come frequently enough this sound is repeated and will be heard as 

 a continuous noise in the telephone (see Fig. 15). This sound can 

 be cut up into dot and dash signals by a key in the sending instru- 

 ment. If the transmitter is sending persistent oscillations, then some 

 form of interrupter has to be inserted in the receiving circuit to enable 

 us to receive a continuous sound in the telephone which can be re- 



