26 



NATURE 



[July 7, 19 10 



to this proportion, the sending voltage would affect the 

 receiver with disturbances equal in size to those due to 

 the receiving voltage ; the duplex would then be very badly 

 indeed out of balance. 



To receive properly, the sending voltage must produce 

 no movement of the receiver whatever; that is to say, 

 any disturbance due to this cause must certainly be less 

 than one-tenth of that due to the arrival current. 



Taking the figures I have given, we see that the balance 

 must be obtained and maintained so that, applying 40 volts 

 to the cable and artificial line, the two currents dividing 

 must not vary more than what will produce 1/200 volt; 

 that is, must be balanced to an accuracy of 8000 to i. 



If, after the duplex has been established, the artificial 

 line varies in its electrical properties as much as 1/8000 

 of its value, the balance would require adjustment so as 

 to keep it useful for receiving. The sensitiveness under 

 these conditions may be considered as equivalent to the 

 sensitiveness of an ordinary metal balance that with 

 8 grams in each pan must turn accurately with i milli- 

 gram. 



It is now found necessary to maintain still more perfect 

 balances for my new method of " high-speed working of 

 cables"; in fact, a balance that must be maintained to 

 within the proportion of 72,000 to i. To do this, the very 

 greatest care has to be directed to questions of insulation 

 and temperature correction, and special appliances are sup- 

 plied to obtain this high degree of accuracy. In fact, 

 the future of " high-speed working of cables " is locked 

 up very much with this question of more delicate and 

 accurate balances ; and if still more perfect balances could 

 be obtained, still higher working speeds of cables would 

 immediately be possible. 



I now come to the instruments employed to work the 

 cables, starting with the sending end. As before pointed 

 out, the various letters of the cable alphabet are com- 

 posed of combinations of -(- and - electrical impulses, 

 or of the records that these impulses produce. The letter 

 e is a -J- impulse, t 3. — one ; a is composed of two 

 impulses, a -\- and -, and so on for all the other letters. 

 The operator has, therefore, first to translate the message 

 to be sent into the cable code, and then to tap on the 

 sending-key the order of the impulses that make up the 

 code message. A sending-key consists of two levers ; the 

 depression by the finger of either one or the other deter- 

 mines which end of the battery, the -t- or - end, is joined 

 to the cable. 



Sending messages by hand is open to two objections : 

 one the want of speed, the other the want of accurate 

 spacing of the letters. A good trained clerk can send at 

 the rate of about 140 letters per minute ; but as most 

 cables are capable of being worked at greater speeds, auto- 

 matic or machine transmission has now become universal. 



An automatic transmitter is an instrument that does 

 the work of the clerk in sending; the two levers of the 

 hand key are now operated upon by mechanism driven by 

 a motor, through the agency of a perforated ribbon. 

 Lveryone who is acquainted with the pianola or auto- 

 matic piano-player knows that the music to be played is 

 punched as holes in a broad paper strip; this strip is run 

 through the machine, and determines which levers are to 

 press upon the keys of the piano. 



The operation of the automatic transmitter is precisely 

 like this, only instead of the extended keyboard there are 

 two keys, a -|- and -, and the paper strip is a narrow 

 ribbon with only two rows of holes to work the levers. 



To send a message, the clerk first of all, bv mearis of 

 a hand perforator, punches the message as combinations of 

 holes in the paper ribbon ; this ribbon, after being per- 

 forated, is fed through the automatic transmitter. 



The automatic transmitter is a motor-driven instru- 

 ment, adapted to feed the perforated ribbon over the ends 

 of a pair of blunt needles. These needles are kept per- 

 petually moving up against and away from the moving 

 ribbon, but if there is a hole in the paper, that particular 

 needle over which it is fed will find it, and the needle 

 will move a little way through the hole. Attached to the 

 two needles are contact levers which connect the cable 

 with one or the other pole of the Sending battery. 



When there are no holes in the paper ribbon, "the needles 

 move up against the paper, the further movement is 



NO. 2123, VOL. 84] 



arrested, and the contact with the battery is not closed, 

 but the battery circuit is closed when there is a hole in 

 the paper, because there is nothing now to block the 

 needle, and the further movement through the hole enables 

 the contact lever to close the battery circuit and thus send 

 the signal. 



The sending levers do one or other of two things : they 

 join the cable to earth (in other words, they short-circuit 

 the cable end) or they disconnect the cable from earth and 

 connect it to the battery, so that the battery may send a 

 signal. At the end of each signal the cable is automatic- 

 ally put to " earth." 



Every signalling impulse due to each hole in the paper 

 is, therefore, divided into two parts, the battery or 

 signalling and the earthing portion. These two portions 

 are adjustable relatively to one another ; when the best 

 relationship has been found, it is maintained at that adjust- 

 ment. The object of earthing the cable after the battery 

 contact is to allow the cable to discharge itself, and thus 

 clear itself for the ne.xt signal. Automatic transmitters 

 constructed on this principle are called " plain " auto- 

 matics, and are in universal use. 



The " curb " was a device applied to an automatic trans- 

 mitter to sharpen the signalling impulse, and thus gain 

 greater definition and increased speed by reversing the. 

 battery at the termination of every battery period. The 

 reverse battery voltage helped to neutralise the charge , 

 already in the cable, and thus discharge the cable in 

 quicker time than by simply earthing the cable, as in the 

 " plain " automatic. 



Unfortunately, the use of the " curb " results in a 

 greater voltage stress on the sending end of the cable, for 

 the reason that the reverse voltage of the " curb " is 

 added to the voltage already in the cable ready to dis- 

 charge, and the rapid reversal of current resulting upon 

 the application of the " curb " is liable to cause " jar " 

 disturbances on the duplex balance. For these reasons 

 " curb " automatics are not now employed. 



Instruments adapted to receive messages at the end of 

 long submarine cables must of necessity work at the 

 highest possible speed that the cable will allow, and are 

 of extreme sensitiveness, and as a consequence are of great 

 delicacy. 



There are two kinds of receivers now commonly 

 employed, viz. the siphon recorder and the " drum " cable 

 relay. The siphon recorder, invented by Lord Kelvin in 

 1867, is an instrument that inks the message as received 

 on a moving band of paper. The " drum " cable relay, 

 by means of an electric contact-making device, brings in a 

 fresh source of energy from a local battery, so that the 

 electric signalling impulses are multiplied many times over 

 in power, and are thus enabled to do many useful things 

 besides inking the message, such as working signalling 

 keys to re-transmit the message on to another line, or to 

 guide the levers of an automatic punching machine to 

 perforate the message. The siphon recorder requires the 

 constant attention of a clerk, the " drum " cable relay 

 does not. 



The siphon recorder consists of a bent glass siphon tube 

 nearly as fine as a human hair. The siphon is suspended 

 by a fine bronze wire; one end of the tube dips in a 

 reservoir of blue aniline ink, the other end can move 

 across the surface of a travelling band of paper, upon 

 which it inks its movement. If the end of the siphon 

 touched the paper, the friction thus introduced would be 

 fatal to the proper working of the instrument, because of 

 the loss of sensitiveness ; it is therefore kept in a state of 

 constant vibration by attaching the tube near its end by 

 means of a silk fibre to an electromagnetic vibrator. The 

 message is thus recorded as a close row of ink dots on 

 the moving paper, and the glass tube is quite free to- 

 swing sideways under the action of the received signals. 



The siphon tube is joined by two silk fibres to a 

 rectangular suspended coil of fine insulated copper wire, 

 which coil hangs in a strong magnetic field. The currents 

 from the cable flow through the wire of the suspended 

 coil, and the reaction of these currents with the magnetic 

 field causes the coil to oscillate to one side or the other, 

 depending upon the direction of the current. The motion 

 of the coil is transmitted by means of the two fibres to 

 the siphon, and thus the signals are recorded as received. 



