Stead// Electric Oscillations in Closed Circuits. 675 



oscillators with reference to one another, to obtain a quantita- 

 tive measure o£ the sensibility of any given oscillation-detector. 

 Thus, for instance, it is generally possible, but not always, to 

 find a position for the closed circuit of the receiver, such that at 

 a certain distance no effect can be detected in that receiving 

 circuit by any oscillation-detector, however sensitive. We 

 may then call this the zero position. If the receiving 

 circuit is moved out of the zero position by turning it 

 through a certain angle round any axis, it will begin to be 

 affected by the distant transmitting circuit, and a quantitative 

 measure of any oscillation-detector can be obtained by noting 

 the angle through which the receiving circuit must be 

 turned, so that good audible signals may just be obtained. 

 Whether the action of the sending on the receiving circuit is 

 due to true electromagnetic radiation or to ordinary electro- 

 magnetic induction seems immaterial. The result in either case 

 is that the receiving circuit is the seat of feeble electrical oscil- 

 lations and the oscillation detector has to detect these if it can. 



Another method is to maintain the receiving-circuit in its 

 position of maximum effect, but to upset the tuning of the re- 

 ceiving-circuit by varying the capacity of the condenser or the 

 inductance of the circuit. This varies the mean-square value 

 of the received current and from the resonance-curve enables 

 us to get the measure of the current or potential-difference 

 which the particular oscillation-detector under test will just not 

 detect. The authors have been employing arrangements of this 

 kind very successfully for a long time past in investigations 

 connected with improvements in the Fleming oscillation- valve. 



It has been found convenient to denote the relative telegra- 

 phic value of detectors by stating the angle in degrees through 

 which the receiving-coil has to be rotated from the zero position 

 that good audible signals can be obtained on the telephone. If 

 a note is made of the value in amperes or milliamperes of the 

 current in the closed transmitting-circuit, this can always be 

 recovered, and if the spark-length and spark-frequency are the 

 same, we can always be sure that the sending circuit is in a 

 constant and similar condition when comparative testsaremade. 



The closed receiving-coil is conveniently made by winding 

 silk-covered copper wire, No. 16 S.W.Gr., on a square 

 mahoganv frame, which can be revolved on pivots carried 

 on a baseboard which can itself be set at any required angle 

 (see fig. 3, p. 676). A divided circle and pointer attached to 

 the frame serve to show the angle through which the frame 

 is rotated. In general appearance it resembles an instrument 

 used in Physical Laboratories under the name of an earth- 

 inductor, for obtaining small induced currents by means of the 

 rotation of a coil in the terrestrial magnetic held. 



2 Z 2 



