"It's like this," Gus explained. "The Brain has a given capacity. The Brain also has an optimal operation speed, a definite rhythm in which it works best. Now, if they feed The Brain too many problems too fast, it results in a shock load, the operations rhythm gets disturbed, efficiency goes down. On the other hand if The Brain works with an under-capacity problem load, that's just as bad. In that case the radioactive pyramidal cells will overheat and decompose. Consequently we must aim at a balanced and an even problems load. That's why these pulsemeters are built into all problem-intake panels for the operators to check upon their speeds. "Take an average problem--rocket ballistics, let's say--parts of it may be as simple as adding two and two and others so bad Einstein would reach for the aspirin from out of his grave. "Now I'll show you how it works; the main power is cut off but there's enough juice left in The Brain's system to make this pulsemeter react; it's even more sensitive than a Geiger-Mueller counter." He surveyed a big switchboard and picked out an outlet marked "Pons Varolis for the plug-in." Then snapped a pair of earphones on Lee's head. "There," he said "you'll both see and hear what it does in a little while." * * * * * A soft glow slowly spread over the slanting screen on top of the machine. A crackling as of static entered the earphones and turned into a low hum. On the left corner of the screen a faint green streak of luminosity crawled over to the right; its light gained in intensity and it began to weave and to dance. Simultaneously the hum became articulate like tickings of a heart only much faster. "Is that the pulse of The Brain?" Lee asked. "No," Gus snorted contemptuously. "The Brain isn't even operating. Nothing moves in The Brain now excepting those ebbing residual currents, too low in power to agitate anything but the amplifiers built into this thing. If these were normal operations with a million impulses per second passing through The Brain you could hear and see as little of the pulse as of the beatings of a million mosquito wings. In that case the dial to your right works a reduction-gear, kind of an inverted stroboscope; that cuts the speed down a hundred-thousand to one and you just barely see and hear the rhythm of the beat." "I see." Fascinated by the dance of the green line Lee said absently, "This touches upon another question I had in mind; The Brain is expanding, that is, new cell groups and circuits are constantly being added. Right?" "Right." "I also understand that The Brain is learning all the time. The cerebral mantle evolves through being worked; its cells enriched by the material submitted to them for processing; the richer the material, the richer their yield. Right?" "Right."