TRANSACTIONS OF SECTION a. 875 



It was thought at one time thatthespeedofwoi'Mng- was limited by the retard- 

 ation of the line circuit, but a careful inquiry into the phenomena of electromag- 

 netic inertia (' Journal ot the Society of Telegraph Engineers,' 1887, vol. v. p. 27) led 

 to the conclusion that the principal source of slowness was in the electromagnet. 

 Every electromagnet has thus a time constant which determines the rate beyond 

 which it cannot work. This time constant can be obtained only by experiment, 

 for it depends on the quality and quantity of iron used, on the form of the core, on 

 the resistance and quality of insulated copper wire, on the number of turns, and on 

 the way they are wound. After numerous experiments it was formd that this 

 eH'ect of the electromagnet could be eliminated by the use of a shunted condenser, 

 and the introduction of this instrument has had the most marvellous eifect on the 

 speed of working. It has entirely eliminated any hindrance caused by the electro- 

 magnet, and now the only cause of slow working is the mechanical efficiency of tlie 

 apparatus and the condition of the circuit. 



Four hundred and tifty words a minute are now obtained with ease on circuits 

 two hundred miles in length, and on some circuits six hundred words per minute 

 are reached ; but four hundred and fifty are more than can practically be coped 

 with, and therefore at present the apparatus exceeds in efficiency the capacity of the 

 staff; but this speed rapidly falls off as the retardation of the circuit increases, and 

 while it is possible to work at the highest speed between London and Leeds, only 

 one-fourth of this speed is practicable to Glasgow. If, however, we place at Leeds 

 a repeater, which will respond to and relay on these frequent currents, we ought 

 to get the maximum speed to Glasgow, and this is done by the high-speed repeater. 

 These instruments are very extensively used. There are special relay offices at 

 Haverfordwest, Nevin, and Anglesey to provide full speed to Ireland. The speed 

 to Ireland in 1870 was fifty words per minute. It is now four hundred and sixty 

 two words — a ninefold advance. Leeds, Manchester, Bristol, and Preston have 

 also special relay rooms, and there are 101 repeaters in use. 



The introduction of high-speed repeaters and the use of shunted condensers have 

 marked epochs in the evolution of telegraphy as eventful as the introduction of 

 duplex working or of the telephone. 



3. Underground Conductors for Electric Lighting, ^c. 

 By Professor G. Forbes, M.A., F.B.S. L. ^- E. 



The author has designed the proposed system to fulfil several important 

 conditions. 



1. The conductors and their insulation should be economical in construction. 



2. They should be protected from injury by a trough or casing. 



.3. This trough should be of small cost and its merits must have been well 

 tested. 



4. The trough must be capable of carrjdng conductors at several different 

 potentials. 



6. It must be possible gradually to add to the conductors as the consumption 

 of electricity in a district increases. 



6. An easy means must be provided for taking branches from the mains into 

 houses. 



7. An easy means must be provided for leading the conductors round gas and 

 water pipes, and other obstacles. 



The first condition can best be secured by having bare copper-wire conductors 

 and air-insulation. The second and third conditions by using ordinary cast-iron 

 gas pipes whose qualities are thoroughly well known, and whose laying and repair- 

 ing and keeping water-tight is everyday work in every town in the country. 



The fourth condition is attained by having porcelain insulating discs, two in 

 each cast-iron pipe. Each insulator has as many holes through it as there are 

 different potentials to be maintained. These porcelain discs are supported on the 

 iron pipes only at a few points, the intervening spaces allowing drainage in tlie 

 cast-iron pipes, and also permitting dry air to be forced through a system of 

 pipes. 



