Electricity and Mechanics

When electric charges are in motion the forces between them differ from the electrostatic forces. The magnetic concept is used to take account of these non-electrostatic forces. The magnetic effect of a moving charge is relative to the electrostatic extremely small except when the velocity approaches that of light. However due to the circumstance that the electrostatic effects often, as in a wire carrying a current, practically cancel, the magnetic forces are very important. According to our present beliefs any field of force changes in an analogous manner if it be moved. There is theoretically the same excuse for speaking of a magnetic field about a moving mass as a moving charge. The difference is that in this latter case the discrepancy from the gravitostatic force is not of practical importance