186 ±)r. B. van der Pohl on the Relation of Audibility 



found to produce very steady oscillations, of which the ampli- 

 tude, wave-length, damping, and number of trains per second 

 could be varied at will within considerable ranges. 



In fig. 1 H is a high tension battery of secondary cells, 

 Oi and CY are two variable air-condensers of approximately 

 equal capacity. The working of the two commutators is such 

 that A is connected to D at the same time that B is connected 

 to F. At this moment the condenser C x ' becomes charged to 

 the voltage of H and any charge on X is discharged through 

 the self-inductance L l5 so that in the circuit C X L X high fre- 

 quency oscillations are set up. At the moment when the 

 commutator has revolved through an angle of 90°, Ci becomes 

 charged and C x ' discharges through the same inductance L x , 

 for at this moment A is connected to E and B to G. The 

 capacity of C/ was varied, so that with its leads and L x it was 

 in tune with C x , its leads, and L x . The leads C X A, C/B, HE, 

 and L X G were made up of 20 strands, each 30 metres long. 

 The commutator, which was driven by an electromotor at a 

 speed of approximately 3500 R.P.M. (467 discharges per 

 second), was not in the same room where L x with the 

 receiver circuit No. 2 was placed, to avoid the noise of 

 the commutator interfering with the small sounds to be 

 heard in the receiving telephone. The inductance L x con- 

 sists of a flat vertical coil of one layer of 15 turns with an 

 inside diameter of 8*5 cm. and an outside diameter of 21 cm. 

 Coil L 2 , whose distance from L x can be varied by known 

 amounts up to 100 cm., has the same dimensions as L x , but 

 consists of two layers. L 2 forms, with the variable air-con- 

 denser C>2, a high frequency circuit from which the signals, 

 produced by tapping the key K, can be heard in the tele- 

 phones T which, in series with a Perikon detector [Chal- 

 copyrite Gu FeS 2 — Zincite ZO], are connected across the 

 terminals of C 2 . A headgear of two Sullivan telephones 

 [resistance R = 1240 O in series] was used. These tele- 

 phones were shunted by a variable resistance S as shown 

 in fig. 1. 



The E.M.F. induced in L 2 is proportional to MI, where M 

 is the mutual inductance between the parallel coils L x and L 2 

 and I the current amplitude in circuit No. 1 ? when only the 

 couplings coefficient & = M/>v/L x L 2 is small enough not to 

 allow for any appreciable reaction from circuit No. 2 on 

 circuit No. 1. Varying therefore the mutual inductance at 

 a constant primary current has the same effect on circuit 

 No. 2 as varying the primary current I at a constant 

 coupling. 



By moving L x to and fro the distance between L x and L 2 



