II. MULTIPLEX TELECEA PHS.—The system of ,Yr. Delany is based upon two main prin ciples : First, that of synchronism, or the simultaneous motion Of similar pieces of apparatus at two different places ; and, secondly. that of distributing to several telegraphists the use of a wire for very short equal periods of time, so that practically each telegraphist has the line to himself during these periods.
The combination of these principles of working by synchronism and multiplex telegraphy on the same wire was first attempted by Moses O. Farmer in 1833. using two wires, one for maintaining in synchronism the distributors which put four operating instruments in con nection with the others. This was introduced by Meyer in 18 ; it was improved upon by Paul In Cour in 1878, and Bandot in 1881; synchronism was perfected by Delany in 1882, and the system completed in 1884, and is now extensively used by the British Postal Tele graph department, where it has reached its highest development under direction of W. H. Preece, F. R S., electrician-in-chief.
The instruments at each station are connected to identical "distributors," consisting of a number of segments arranged in a circle over which travels an arm. If each segment be divided into four segments, and by means of these be connected with, say, four instruments instead of with only one of them, then during one complete rotation each arm will place cor responding instruments in communication with each other four times. Or if each circle be divided into 40 segments, and each of these into four segments, then corresponding instruments will be in communication with each other forty times during each complete rotation of the arms. In the British post-office apparatus there arc 168 segments, and these are grouped differently, according to the munber of ways of working. Sextuples work_ng requires one grouping, quadruples another, triplex another, and so on.
Two tuning forks pitched to absolutely the same note, and set in vibration by currents like an electric trembling bell, will move in synchronism, but the synchronism can not be maintained. The deposition of dirt, dust, or moisture, changes of temperature, variation of current, produce changes which affect the rate of motion. Paul la Cour, of Copenhagen,
invented an ingenious way to maintain the synchronism, the principle of which has been introduced into the Delany system. A simple reed is now used as the means of keeping the distributor in synchronous motion. The eleetro-magnet of the reed is wound to a resistance of 30 ohms. Its local circuit includes the lever and lower contact of a relaying sounder. The correction for synchronism of the two revolving arms is effected by causing tins lever to rise, thus breaking tile circuit of the reed magnet when a correcting current is received.
The distributing circle is divided into 168 equal spaces, furnisned with segments insulated from each other ; 114 of these segments are connected to form twelve groups for telegraphing, the remaining spaces being fitted with segments for synchronizing purposes. Segments 1 and 1, 2 and 2, 3 and 3, anc' so on, are electrically connected together to form the groups, and each group of twelve segments thus arranged is connected to a terminal on the base of the distributor. An arm, or trailer, passes lightly over the surface and moves continuously round the circle, coming successively in contact with every segment, moving in the opposite direction to that of the hands of a watch. It is electrically connected to the line wire. In every rotation it makes 168 electrical contacts, 144 of which are for telegraphing, while the others are for maintaining synchronism.
The function of the trailer is to place the line wire successively in connection with the segments in the different groups. The currents of electricity that flow through the line wire are dependent upon the operations performed upon the telegraphic apparatus, and they MO broken up into short pulsations or impulses by the momentary contact made by the trailer. The relay is of the standard form. but it is touch larger ; its cores are nearly 4 in. long by 1 in. in diameter, and it is wound to a resistance of 1,200 ohms with copper wire in. in diameter.