repulsive; in all cases, however, the same law applies as to the action through space.
The reader should note that the above statement of this law is no complete answer to the question as to the actual magnitude of the force. Experiment only teaches us that the force is proportional to the product of the two masses divided by the square of their distance, but if we wish to state the actual magnitude of this force in a definite figure we must agree on a system of units. As far as the attractive force between ponderable masses is concerned, such units are quite familiar; we know what is meant by the mass of a pound weight, and we also know how to measure a distance. With magnetic and electric forces the matter is not so simple. A distance we can measure in any length unit, but what about the unit for the "active mass"? We have seen that it is not a mass at all in the common acceptance of this term, and it can therefore not be expressed in any unit suitable for ponderable masses. We are thus compelled to settle the magnitude of the unit by the same formula which defines the force. The conception of unit active mass may then be derived from the following condition: If two equal masses one centimetre
