matter of the opposite polarity. Another difficulty lies in this, that the magnetic matter is spread over the whole of the bar—more dense at the ends, but still of sensible density at points towards the middle. Thus it becomes difficult to estimate the average distance of action. These difficulties are so great that the same methods of experimenting, which we supposed to be used when investigating electric forces, become quite impracticable, and other methods have to be devised.
These methods are based on the conception of the magnetic moment, that is, the product of the distance of poles into their strength. Any physical magnet can then be considered as a bundle of mathematical magnets, each carrying magnetic matter only at the extreme ends. We observe experimentally the summarised effect of all these elementary magnets, and by mathematical reasoning we are able to deduce the law under which magnetic forces act across space. Experiment shows that the law stated in the beginning of this chapter also holds good for magnetic forces. Moreover, the magnitude of the unit of magnetism may be determined in the same way. If we find that two equally strong poles placed one cm. apart exert on each
