_Static Electricity._—This is a term employed to define electricity produced by friction. It is properly employed in the sense of a static charge which shows itself by the attraction or repulsion between charged bodies. When static electricity is discharged, it causes more or less of a current, which shows itself by the passage of sparks or a brush discharge; by a peculiar prickling sensation; by an unusual smell due to its chemical effects; by heating the air or other substances in its path; and sometimes in other ways. _Current Electricity._—This may be defined as the quantity of electricity which passes through a conductor in a given time—or, electricity in the act of being discharged, or electricity in motion. An electric current manifests itself by heating the wire or conductor, by causing a magnetic field around the conductor and by causing chemical changes in a liquid through which it may pass. NOTE.—_Statics_ is that branch of mechanics which treats of the forces which keep bodies at rest or in equilibrium. _Dynamics_ treats of bodies in motion. Hence static electricity is electricity at rest. The earth’s great store of electricity is at rest or in equilibrium. _Radiated electricity_ is electricity in vibration. Where the current oscillates or vibrates back and forth with extreme rapidity, it takes the form of waves which are similar to waves of light. _Positive Electricity._—This term expresses the condition of the point of an electrified body having the higher energy from which it flows to a lower level. The sign which denotes this phase of electric excitement is +; all electricity is either positive or,-, negative. _Negative Electricity._—This is the reverse condition to the above and is expressed by the sign or symbol-. These two terms are used in the same sense as _hot_ and _cold_. _Atmospheric electricity_ is the free electricity of the air which is almost always present in the atmosphere. Its exact cause is unknown. The phenomena of atmospheric electricity are of two kinds; there are the well-known manifestations of thunderstorms; and there are the phenomena of continual slight electrification in the air, best observed when the weather is fine; the aurora constitutes a third branch of the subject. _Dynamic Electricity._—This term is used to define current electricity to distinguish it from static electricity. This is the electricity produced by the dynamo. _Frictional electricity_ is that produced by the friction of one substance against another. _Resinous Electricity._—This is a term formerly used, in place of negative electricity. The phrase originated in the well known fact that a certain (negative) kind of electricity was produced by rubbing rosin. _Vitreous electricity_ is a term, formerly used, to describe that kind of electricity (positive) produced by rubbing glass.