{ magnetoscope, dip, { electroscope, electron, { declination, pole, { positive, negative, { retentivity, { potential, capacity, Terms: { permeability, { condenser, electrophorus, { lodestone, { oscillatory discharge, { magnetic meridian. { lightning. Likeness: { _a_--produced by induction, _b_--attract both are: { and repel, _c_--have fields of force. { _a_--electricity can be _conducted_, { magnetism cannot. { Differences: { _b_--electricity in _all substances_, { magnetism in few. { { _c_--magnetism with the compass indicates { direction. CHAPTER XI CURRENT ELECTRICITY (1) ELECTRICAL CURRENTS AND CIRCUITS =237. Sources of Electric Currents.=--In studying the production and distribution of static electricity it was seen that if two bodies at _different potentials_ are connected by a copper wire a _movement of electricity to the body_ having the _lower potential_ occurred along the conducting wire. This movement of electricity is called an _electric current_ (Art. 227). _A difference of potential_ is therefore often called an _electromotive force_ (E.M.F.), since it produces the movement of electricity in a conductor. The current between two _oppositely charged_ bodies lasts for so short a time as to be of little or no practical value unless some means are found for continually recharging the bodies. That is, some device must be used to restore the difference in potential as fast as the conducting wire equalizes it. The continual charging of the bodies takes work. In other words, it requires a continual expenditure of some form of energy (which is converted into electrical energy) to produce the electric current. Two forms of energy are commonly used for this purpose. (A) _Chemical energy_ is employed in _voltaic cells_ for producing electric currents. (B) _Mechanical energy_ is used for the same purpose in the _dynamo_ and similar devices.