and
other oddments lying around the lab. As his tinkering didn't interfere with his work, no-one objected, indeed, the physics demonstrators did their best to encourage him, because, after all, there is something refreshing about any form of enthusiasm. But no-one expected he'd ever get very far, because I don't suppose he could even integrate e to the x.'
as such ignorance possible?' gasped someone.
'Maybe I exaggerate. Let's say x e to the x. Anyway, all his knowledge was entirely practical-rule of thumb, you know. Give him a wiring diagram, however complicated, and he could make the apparatus for you. But unless it was something really simple, like a television set, he wouldn't understand how it worked. The trouble was, he didn't realize his limitations. And that, as you'll see, was most unfortunate.
'I think he must have got the idea while watching the Honours Physics students doing some experiments in acoustics. I take it, of course, that you all understand the phenomenon of interference,?'
'Naturally,' I replied.
'Hey!' said one of the chess-players, who had given up trying to concentrate on the game (probably because he was losing). 'I don't.'
Purvis looked at him as though seeing something that had no right to be around in a world that had invented penicillin.
'In that case,' he said coldly, 'I suppose I had better do some
explaining.' He waved aside our indignant protests. 'No, I insist.
It's precisely those who don't understand these things who need to
be told about them. If someone had only explained the theory to
poor Fenton while there was ' s still time… 1,
He looked down at the now thoroughly abashed chess-player.
'I do not know,' he began, 'if you have ever considered the nature of sound. Suffice to say that it consists of a series of waves moving through the air. Not, however, waves like those on the surface of the sea-oh dear no! Those waves are up and down movements. Sound waves consist of alternate compressions and rarefactions.'
'Rare-what?'
'Rarefactions.'
'Don't you mean 'rarefications'?'
'I do not. I doubt if such a word exists, and if it does, it shouldn't,' retorted Purvis, with the aplomb of Sir Alan Herbert dropping a particularly revolting neologism into his killing-bottle. 'Where was I? Explaining sound, of course. When we make any sort of noise, from the faintest whisper to that concussion that went past just now, a series of pressure changes moves through the air. Have you ever watched shunting engines at work on a siding? You see a perfect example of the same kind of thing. There's a long line of goods-wagons, all coupled together. One end gets a bang, the first two trucks move together-and then you can see the compression wave moving right along the line. Behind it the reverse thing happens-the rarefaction-I repeat, rarefaction-as the trucks separate again.
'Things are simple enough when there is only one source of sound-only one set of waves. But suppose you have two wave patterns, moving in the same direction? That's when interference arises, and there are lots of pretty experiments in elementary physics to demonstrate it. All we need worry about here is, the fact-which I think you will all agree is perfectly obvious-that if one could get two sets of waves exactly out of step, the total result would be precisely zero. The compression pulse of one sound wave would be on top of the rarefaction of another-net result-no change and hence no sound. To go back to my analogy of the line of wagons, it's as if you gave the last truck a jerk and a push simultaneously. Nothing at all would happen.
'Doubtless some of you will already see what I am driving at, and will appreciate the basic principle of the Fenton Silencer. Young Fenton, I imagine, argued in this manner. 'This world of ours,' lit said to himself, 'is too full of noise. There would be a fortune for anyone who could invent a really perfect silencer. Now, what would that imply… T
'It didn't take him long to work out the answer: I told you he was a bright lad. There was really very little in his pilot model. It consisted of a microphone, a special amplifier, and a pair of loudspeakers. Any sound that happened to be about was picked up by the mike, amplified and inverted so that it was exactly out of phase with the original noise. Then it was pumped out of the speakers, the original wave and the new one cancelled out, and the net result was silence.
'Of course, there was rather more to it than that. There had
to be an arrangement to make sure that the canceling wave was just the right intensity-otherwise you might be worse off than when you started. But these are technical details that I won't bore you with. As many of you will recognize, it's a simple application of negative feed-back.'
'Just a moment!' interrupted Eric Maine. Eric, I should mention, is an electronics expert and edits some television paper or other. He's also written a radio play about space-flight, but that's another story. 'Just a moment! There's something wrong here. You couldn't get silence that way. It would be impossible to arrange the phase…
Purvis jammed the pipe back in his mouth. For a moment there was an ominous bubbling and I thought of the first act of 'Macbeth'. Then he fixed Eric with a glare.
'Are you suggesting,' he said frigidly, 'that this story is untrue?'
'Ah-well, I won't go as far as that, but Eric's voice trailed away as if he had been silenced himself. He pulled an old envelope out of his pocket, together with an assortment of resistors and condensers that seemed to have got entangled in his handkerchief, and began to do some figuring. That was the last we heard from him for some time.
'As I was saying,' continued Purvis calmly, 'that's the way Fenton's Silencer worked. His first model wasn't very powerful, and it couldn't deal with very high or very low notes. The result was rather odd. When it was switched on, and someone tried to talk, You'd hear the two ends of the spectrum-a faint bat's squeak, and a kind of low rumble. But he soon got over that by using a more linear circuit (dammit, I can't help using some technicalities!) and in the later model he was able to produce complete silence over quite a large area. Not merely an ordinary room, but a full-sized hall. Yes….
'Now Fenton was not one of these secretive inventors who won't tell anyone what they are trying to do, in case their ideas are stolen. He was all too willing to talk. He discussed his ideas with the staff and with the students, whenever he could get anyone to listen. It so happened that one of the first people to whom he demonstrated his-improved Silencer was a young Arts student called –I think-Kendall, who was taking Physics as a subsidiary subject.
Kendall was much impressed by the Silencer, as well he might be. But he was not thinking, as you may have imagined, about its commercial possibilities, or the boon it would bring to the outraged ears of suffering humanity. Oh dear no! He had quite other ideas.
'Please permit me a slight digression. At college we have a flourishing Musical Society, which in recent years has grown in numbers to such an extent that it can now tackle the less monumental symphonies. In the year of which I speak, it was embarking on a very ambitious enterprise. It was going to produce a new opera, a work by a talented young composer whose name it would not be fair to mention, since it is now well-known to you all. Let us call him Edward England. I've forgotten the title of the work, but it was one of these stark dramas of tragic love which, for some reason I've never been able to understand, are supposed to be less ridiculous with a musical accompaniment than without. No doubt a good deal depends on the music.
'I can still remember reading the synopsis while waiting for the curtain to go up, and to this day have never been able to decide whether the libretto was meant seriously or not. Let's see-the period was the late Victorian era, and the main characters were Sarah Stampe, the passionate postmistress, Walter Partridge, the saturnine gamekeeper, and the squire's son, whose name I forget. It's the old story of the eternal triangle, complicated by the villager's resentment of change-in this case, the new telegraph system, which the local crones predict will Do Things to the cows' milk and cause trouble at lambing time.
'Ignoring the frills, it's the usual drama of operatic jealousy. The squire's son doesn't want to marry into the Post Office, and the gamekeeper, maddened by his rejection, plots revenge. The tragedy rises to its dreadful climax when poor Sarah, strangled with parcel tape, is found hidden in a mail-bag in the Dead Letter Department. The villagers hang Partridge from the nearest telegraph pole, much to the annoyance of the linesmen. He was supposed
