The electrical force which moves the electricity in the circuit, just described, experiences two principal reactions. One reaction is due to the lines of electrical force which, attached to the electrical charge on the condenser plates, are crowded into the insulating space between these plates. This reaction corresponds to the elastic reaction of the prongs of the tuning-fork, and follows the same law. In the case of the tuning-fork the _elastic reaction is proportional to the displacement_ of the prongs from their normal position; in the electrical case the reacting force is proportional to the electrical charges which have been pulled apart, the negative from the positive, and driven to the plates of the condenser. Call this separation _electrical displacement_, and the law can be given the same form as above, namely: _The reacting force is proportional to the electrical displacement_. The greater the distance between the plates, and the smaller their surface, the greater is the reaction for a given electrical displacement. By varying these two quantities we can vary the electrical yielding, the so-called capacity, of the electrical condenser. This is what you do when you turn the knob and vary the capacity of the condenser in your receiving set. The moving prongs have a momentum, and a change in the momentum opposes a reacting force, the so-called inertia reaction, which is equal to the rate of this change. This was discovered by Galileo over three hundred years ago. We experience the operation of this law every time we bump against a moving object. The Irish sailor who, after describing the accident which made him fall from the mast, assured his friends that it was not the fall which hurt him but the sudden stop, appreciated fully the reacting force due to a rapid change of momentum. Every boy and girl in the public schools should know Galileo’s fundamental law, and they would know it if by a few simple experiments it were taught to them. But how many teachers really teach it? How many of my readers really know that law? Just think of it, what an impeachment it is of our modern system of education to have so many intelligent men and women, boys and girls, ignorant of so fundamental a law as that which Galileo discovered so long ago!