1909 J on Modern Suhmarlne Telegraphy. 531 



varies up to a maximum of from 20 to 40 henries, and its weight from 

 1 to 3 cwt. 



In the case of a siphon recorder used as the receiver, the shunt 

 short-circuits the suspended coil and the series condenser is 

 aboUshed. 



In the case of a cable relay, the series condenser is usually retained 

 to insure that earth currents are effectually stopped, but the condenser 

 is made large. 



A shunt inductance has a similar time action on the in-coming 

 current to that of a series condenser, but with this improvement — 

 that it helps to reduce the phase distortion of current with voltage 

 rather than accentuate it, as is the case with the condenser. 



Having obtained the best value of the shunt alone, the following 

 curious effect was discovered : that adding a condenser as an 

 additional shunt, the size of the signals on the recorder got larger and 

 more distinct. The mathematical reason for this is as follows : 

 that for any particular frequency, say the highest frequency of the 

 cable signalling, the shunts of inductance and capacity when properly 

 proportioned act as a shunt of infinite resistance. For frequencies 

 much below this it is as if we had no condenser at all. For frequen- 

 cies much above this it is as if we had no inductance, but only a con- 

 denser. 



To still further reduce the harmful effect of phase displacement 

 series inductances have lately been introduced at the ends of cables, 

 particularly at the sending end. 



By placing an inductive coil of low resistance in series with the 

 battery, at the apex of the duplex bridge, not only has the speed of 

 signalling been increased, but the effect of what is known as " jar " 

 on the duplex balance has also been greatly reduced. 



Before proceeding to describe the instruments that work the 

 cables, I will say a few words about " duplexing." 



All cables are now duplexed, that is to say, are arranged so that 

 messages can be sent and received, at the same time, at each end 

 simultaneously. 



The first cables were duplexed by Stearns and later ones by 

 Muirhead and Taylor. 



Duplex reduces the speed of simplex, or of working one way only, 

 by 20 per cent, but the total carrying power of the cable, irrespective 

 of direction, is raised by some 70 per cent, and is for this reason 

 valuable, and repays the trouble in maintaining the balance. 



Cables are duplexed by arranging an artificial or imitation cable, 

 which is an exact electrical copy of the real, in parallel with the real 

 cable. 



The current from the sending battery flows through two equal 

 arms of capacity or inductance of a Wheatstone bridge arrangement, 

 and into the real and artificial cables. 



The inductive or magnetic bridge which I have applied lately 



