676 Steady Electric Oscillations in Closed Circuits. 



If a closed transmitting-circnit is set up at a distance, 

 then it is generally possible to find a position for the 



Fig. 3. — Radiotelegraphic Detector Tester. 

 receiving-coil, snch that it will not detect any signals when 

 coupled with a condenser and tuned and associated with a 

 highly sensitive receiver. On turning the coil through a 

 certain angle the signals will be heard. If a very sensitive 

 oscillation-detector is employed, then there may be no position 

 of absolutely null reception, but there will be a position of 

 minimum reception. Thus, for instance, in a certain case, 

 with a coil used at the Pender Electrical Laboratory, some 

 Fleming oscillation-valves of a new type were found to be so 

 sensitive to oscillations, that no position in which the receiving- 

 circuit could be placed was so completely a position of zero 

 mutual induction that these valves, when used with a tele- 

 phone, gave no signals from a tuned transmitter. Such 

 valves were called zero valves. Others, on the contrary,, 

 could not detect signals until the coil had been turned through 

 5°, 10°, or 20° from the zero or minimum position. A 

 magnetic detector inserted in series with the coil could not 

 detect the signals from the transmitter until the coil was 

 turned through 15°. An electrolytic detector of a particular 

 make required a rotation of 40°, and a carborundum detector 

 required 45° rotation of the coil to give audible signals on 

 the telephone. These measurements are not given as 

 absolute and final measurements of the relative sensibility 

 of all magnetic, electrolytic, or crystal detectors, but merely 

 as examples of the ease with which the sensibility of these 

 special samples of receivers could be tested for order of 

 sensibility. The instrument has proved of great use in a 

 research being conducted now in connexion with improve- 

 ments in onized gas radiotelegraphic detectors. 



