In the preceding chapter I have hastily alluded to the phenomenon known as the key to electricity as a utilitarian science; a means of material usefulness. These uses are all made possible under the laws of what we term INDUCTION. To comprehend this remarkable feature of electric action, it must first be understood that all electrical phenomena occur in what has been termed an "_Electrical Field_" This field may be illustrated simply. A wire through which a current is passing _is always surrounded by a region of attractive force_. It is scientifically imagined to exist in the form of rings around the wire. In this field lie what are termed "lines of force." The law as stated is that the lines in which the magnetism produced by electricity acts _are always at right angles with the direction in which the current is passing_. Let us put this in ordinary phrase, and say that in a wire through which a current is passing there is a magnetic attraction, and that the "pull" is always _straight toward the wire_. This magnetism in a wire, when it is doubled up and multiplied sufficiently, has strong powers of attraction. This multiplying is accomplished by winding the wire into a compact coil and passing a current through it. If one should wind insulated wire around a core, or cylinder, and should then pull out the cylinder and attach the two ends of the wire to the opposite poles of a battery, when the current passed through the coil the hollow interior of it would be a strong magnetic field. The air inside might be said to be a magnet, though if there were no air there, and the coil were under the exhausted receiver of an air-pump, the effect would be the same, and the _vacuum_ would be magnetized. A piece of iron inserted where the core was, would instantly become a magnet, and when the insulated wire is wound around a soft iron core, and the core is left in place, we have at once what is known as an _Electro-Magnet_. The wire windings of an electro-magnet are always insulated; wound with a non-conductor, like silk or cotton; so that the coils may not touch each other in the winding and thus permit the current to run off through contact by the easiest way, and cut across and leave most of the coil without a current. For it may as well be stated now that no matter how good a conductor a wire may be, two qualities of it cause what is called "_resistance_"--the current does not pass so easily. These two qualities are _thinness_ and _length_. The current will not traverse all the length of a long coil if it can pass straight through the same mass, and it is made to go the long way _by keeping the wires from touching each other_--preventing "contact," and lessening the opportunity to jump off which electricity is always looking for.