There is still another way of generating electricity besides those that we have named; which are friction, chemical action, and the magneto-electric mode of generating a current. Electricity may be generated by heat. If we connect antimony and bismuth bars together and apply heat at the junction of the metals and then connect the free ends of the two bars to a galvanometer, it will indicate a current. These pairs can be multiplied, and in this way increase the voltage or pressure, and, of course, increase the current, if we assume that there is resistance in the circuit to be overcome. If there were absolutely no resistance in the circuit--a condition we never find--there would be no advantage in adding on elements in series. Substances differ in their resistance to the passage of electricity--the less the resistance the better the conductor. The German electrician, G. S. Ohm (1789-1854), investigated this and propounded a law upon which the unit for resistances is based, and this unit takes his name and is called the "ohm." Any two metals having a difference of potential will give the phenomena of thermo-electricity. Antimony and bismuth having a great difference of potential are commonly used. The use made of thermal currents is chiefly for determining slight differences of temperature. An apparatus called the thermo-electric pile has been constructed out of a great number of pairs of antimony and bismuth bars. This instrument in connection with a galvanometer makes a most delicate means of determining slight changes of temperature. If one face of a thermopile is exposed to a temperature greater than its own, the needle will move in one direction; if to a temperature lower than its own, the needle will be deflected in the opposite direction. If both faces of the pile are exposed to the same changes of temperature simultaneously, of course no electrical manifestations will occur. The earth is undoubtedly a great thermal battery that is kept in action by the constant changes of temperature going on at the earth's surface, caused by its rotation every twenty-four hours on its axis. The sun, of course, is at some point heating the earth, which at other points is cooling, making a constant change of potential between different points. If we heat a metal ring at one point a current of electricity will flow around it--especially if it is made of two dissimilar metals--until the heat is equally distributed throughout the ring.