ADVANCES IN SIGNALLING 239 



carry on radio communication and are entirely invisible to the 

 eye are from, say 20 meters to 20 kilometers in, length. A very 

 valuable property of such long waves is that they bend around, 

 and conform to, the spherical surface of land and sea ; whereas 

 short and visible waves, except in extreme cases, maintain 

 straight lines in their advance. If, however, the eye responded 

 to radio waves, we might expect to perceive the compass bear- 

 ing and direction of their source, however remote it might be. 

 Since the human eye is unresponsive, an artificial eye has to be 

 resorted to, which shall enable by its indications the electrical 

 bearing and direction of the source of any radio waves that may 

 be received. Such an instrument is a radio-goniometer, or 

 radio direction finder. 



A goniometer may be mounted on the roof of a small radio 

 house or on the roof of a traveling radio car, or fixed on an 

 airplane. The goniometer in any case consists of a square 

 wooden frame of insulated wire pivoted on a vertical axis, 

 and rotatable by an observer underneath and inside the house. 

 The observer connects the ends of this rotatable coil to a tuning 

 condenser and a sensitive vacuum-tube amplifier. He then lis-^ 

 tens for signals in his head telephone. 



When the plane of the frame coil is parallel to the radio 

 wave front, the radio waves passing the frame produce no 

 electric disturbance in the observer's circuit, and no sound in 

 his ear. On the other hand, when the frame coil is set perpen- 

 dicular to the arriving radio waves, the electric disturbance 

 and telephonically received sound will be a maximum. One 

 way of getting the bearing of the radio station, which is sending 

 the signals, is to rotate the frame until the sounds in the tele- 

 phone pass through zero. The frame is then perpendicular to 

 the direction of the station sought, and the observer can read 

 off the direction from a horizontal disk at the foot of the frame 

 spindle. By practice, an observer can locate, in this way, the 

 electric bearing or direction of a radio station, within a certain 

 small angle of uncertainty. In the case of a powerful station, 

 only a few kilometers away, he can perhaps assign the direction 



