Theory of Metallic Conduction

Author: J.B. Hoag

Metals, such as copper, silver, brass, mercury, tin, zinc, and iron, will conduct an electric current whereas wood, rubber, bakelite, amber, sulfur, textiles, resins, and the like, do not. Solutions of salts and acids are fairly good conductors, and gases at reduced pressures also pass an electric current. Of course there are no perfect conductors, nor are there any perfect insulators, but in many cases, the relative conducting or insulating ability is very pronounced.

In the case of metals, the atoms are so arranged in their space-lattices as to properly influence each other, and contain outer electrons which are so loosely bound to the atom that, on the average, one or two such electrons of each atom escape into the intervening space between the bound atoms. These semi-free electrons move about between the atoms at comparatively high speeds, comparable indeed to that of a small caliber rifle bullet. The metal as a whole is electrically neutral because the negative charge of the free electrons is counterbalanced by the positive charge of the atoms from which they have temporarily escaped. The atoms from which insulators are made are of such an internal structure, and interact upon each other in such a manner, that practically no electrons can escape to move about between them.

A free electron in a metal will travel past approximately one hundred atoms (0.000,001 cm.) before "colliding" with other particles or entering an atom. Thus the free electrons are in a continuous state of movement or thermal agitation; they move about in tortuous paths, and on occasion enter or leave the fixed atoms. Because of these motions, the "cloud" of free electrons contains some which are going fast and some which are going slowly, with all gradations between.

When a battery is connected to the ends of a wire, say a copper wire, the free electrons drift slowly down the wire while they continue their rapid random motions. Suppose a drop of water were to fall into a brimful lake. Immediately, a little wave would travel across the water and shove out a drop of water at the spillway. Later, perhaps, the original drop might wend its way across the lake and leave it. Similarly in the wire, immediately upon connecting the battery, an impulse of electricity travels to the other end of the wire, at just a little less than the velocity of light (which is 186,000 miles per second). Conceivably the drift motion of an individual free electron, which amounts to only a few centimeters per second, might carry it to the other end of the wire in due time.

Over a long interval of time, say one second, the thermal movement of the free electrons is quite uniform. But during a very short time interval, the clouds of free electrons in one part of the wire are denser than in other parts, having slightly larger numbers in a unit volume than the average. Although these variations from the mean value are very small, nevertheless they can be detected. They actually serve as a source of noise in high gain amplifiers. This so-called thermal noise depends on the temperature and electrical resistance of the wires in the input end of the amplifier and on the frequency band which is being amplified.