Just as one teleprinter sends impulses to another teleprinter, so it can send them to an electronic computer. Often, however, it does this not directly, but with the helpful mediation of a punched tape.
Many teleprinters are arranged so that they do not merely send pulses; they can punch holes in paper tape.
For every "1" they make a hole. Where there is an "0", the paper remains untouched. The tape for the sentence "Mary is 17"' is shown here.
Apart from the teleprinter, of course, which works as a tape punch, a "punched-tape sensing element" or "feeler" is also needed to reconvert the holes in the tape into electric impulses, for the electronic computer finds perforated paper ribbons completely indigestible. In its simplest form, a sensing element consists of five pairs of electrical contacts, between which the paper ribbon is drawn. Where there is a hole in the tape, two contacts meet and allow an electric impulse to pass through.
Where there is no hole, of course, the contacts are kept apart.
More sophisticated kinds of punched-tape sensing elements make use of photo-electric cells which transmit current as soon as light falls on them. Five such electric eyes keep watch on the punched tape as it glides past.
Underneath is a lighted lamp which sends a beam of light where every hole appears. The position of the hole causes the light to hit the appropriate photo-electric cell which in turn transmits a pulse.
The question now naturally arises why it was found necessary to make use at all of this complicated - and certainly not very cheap - roundabout method using punched tapes. Why couldn't the teleprinter feed the electrical pulses direct to the computer? (In many smaller installations it actually does so.) The answer is simple: this seemingly expensive detour saves money.
An automatic calculator can accept many thousands of symbols (numbers or letters) per second. The man at the teleprinter, however, can type only four to eight characters in the same time. It would be unprogressive, and certainly unprofitable, to slow down a computer which had been purchased because of its speed, to the snail's pace of the teleprinter operator. The speed of a punched-tape sensing element suits the nature of the automatic calculator much better. A photo-electric cell contactor can read off 400 to 1.000 symbols per second from the punched tape, and transfer them to the computer in the form of pulses. So, hours can be spent in typing whole bundles of punched tapes for the computer to receive in a few minutes - even in seconds.
While the computer is processing them, new tapes can be punched out for it.
As a matter of fact, the process takes place in an even more complicated fashion, as the pulses stamped into the paper are transferred first of all to a magnetic tape and from it to the computer. Magnetic tapes are similar to the ones used in ordinary tape-recorders, only wider.
Their minute magnetic particles can record sound waves, and in just the same way, the tapes store the bits after they have been converted into pulses.
But why this detour via the magnetic tape? As in the case of the punched tapes, for speed's sake. Magnetic tapes can deliver magnetized pulses so quickly that the computer can be supplied in one second with 100.000 characters. Can you visualize 100.000 characters? A thousand hundreds, or the number of individual letters in about forty pages of an average book. And, in one second! a stretch of time in which ordinary mortals like us can just manage to count up to eight. At such speeds, of course, great difficulty crops up when the tape is started or stopped. So, stopping must be done with a sharp jolt every time, lasting no more than a few thousandths of a second, so that no magnetized information accidentally drops out as a result of overly slow stopping or starting.
Punched tapes and recording tapes are not the only means of making contact with an electronic computer.
We must not forget the good old-fashioned punchedcard. It was invented three hundred years ago, for controlling mechanical looms. Herman Hollerith, an engineer and statistician from Washington, D.C., rediscovered it 130 years ago when he was up against the task of evaluating 50 million census questionnaires. He had the answers in each questionnaire perforated, according to a simple code, on stiff cards. With the aid of mechanical and electrical devices, the cards could then be sorted to give selected information according to a predetermined classification. All inhabitants between the ages of 30 or 40, for instance, or all butchers, bakers and candlestick-makers, could be picked out. If it was desired, the sorting machines could be set so that of all the 50 million cards only the 42-year-old unmarried butchers in Nevada would come to light.
Punched cards work on the same lines today. Many industries record addresses, orders, deliveries and accounts on punched cards. Hollerith machines store and arrange the cards, evaluate them and make calculations. For complicated jobs of the kind that one would prefer to entrust to an electronic computer, the cards are stacked into a "punched-card reader" which senses them with light rays and photo-electric cells, translates their perforated script into the pulse language of the computer, and leads them to the automatic calculators. Such a device can read up to 120.000 punched cards in an hour.
We just mentioned that the punched-card pulses must be translated into the machine language of the computer. Interpreting assignments of this kind are frequent in the world of electronic calculation. Up to now, we have written as if there were no other code than a teleprinter code. We found this assumption very practical, but unfortunately the true facts are somewhat different. There are dozens of codes suitable for use in one department or another of electronic computing - codes with four, five, seven, eight, ten, fifteen or forty channels. Even among the five-channel codes alone, to which the teleprinter language belongs, we find variants, and we shall have to deal with one of them in the next chapter. Every firm cherishes the code it uses (often the code it has invented) just as the Cockney is attached to his own way of speaking English. Fortunately, there are such things as translation machines capable of turning one "dialect" - that is, one code-into another, exactly as the teleprinter converts our ordinary letters of the alphabet into its own code. Coupling-in such apparatus makes it possible to connect computers, speaking in a code that is all their own, to the teleprinter network, or to make the computers read punched cards that again have their own distinctive code.
We might mention that the punched card has recently evolved into a "magnetic card". On it, the symbols are no longer marked in the form of holes but - as in the magnetic tape - by magnetized pulses.