Leray says that from his own experience upon pupils from eleven to thirteen years old — “gives even to the dullest children the taste or even the passion for physical experiment.”
Take, for instance, the description of Atwood’s machine in any course of elementary physics. You will find very great attention paid to the wheels on which the axle of the pulley is made to lie; hollow boxes, plates and rings, the clock, and other accessories will be mentioned before one word is said upon the leading idea of the machine, which is to slacken the motion of a falling body by making a falling body of small weight move a heavier body which is in the state of inertia, gravity acting on it in two opposite directions. That was the inventor’s idea; and if it is made clear, the pupils see at once that to suspend two bodies of equal weight over a pulley, and to make them move by adding a small weight to one of them, is one of the means (and a good one) for slackening the motion during the falling; they see that the friction of the pulley must be reduced to a minimum, either by using the two pairs of wheels, which so much puzzle the text-book makers, or by any other means; that the, clock is a luxury, and the “plates and rings” are mere accessories: in short, that Atwood’s idea can be realised with the wheel of a clock fastened, as a pulley, to a wall, or on the top of a broomstick secured in a vertical position.
In this case the pupils will understand the idea of the machine and of its inventor, and they will accustom themselves to separate the leading idea from the accessories; while in the other case they merely look with curiosity at the tricks performed by the teacher with a complicated machine, and the few who finally understand it spend a quantity of time in the effort. In reality, all apparatus used to illustrate the fundamental laws of physics ought to be made by the children themselves.