sure. latter, and E, the pressure at the tap. The quantity flowing out is proportional to E-E2 = E. This difference of pressure, which may be represented simply by E, may be called the watermotive force. Similar conditions exist in the phenomenon of electromotive force, or electric presAll that the battery or dynamo does is to set up a difference of potential, or difference of electric pressure between two points, called electromotive force E. The phenomenon of the equalization of this difference in a circuit is what we mean by current flow. Now, if the opposing forces, or resistance, be unity, the unit of E.M.F. is that which will produce unit flow of current through the circuit. The absolute unit of force being very small, the practical unit of E.M.F. is chosen equal to 100,000,000 or 10o C.G.S. units, and is called the volt. The International Congress, mentioned before, recommended for adoption, As the unit of electromotive force, the international volt, which is the E.M.F. that steadily applied to a conductor whose resistance is one international ohm, will produce a current of one international ampere, and which is represented sufficiently well for practical use by 100 of the E.M.F. between the poles of the voltaic cell, known as Clark's Cell, at a temperature of 15° C., and prepared according to specification. 1434 The Clark cell has therefore 1.434 international volts of E.M.F. The Carhart cell, which is much superior to the old Clark, is now much used in this country as a standard. It has an E.M.F. of 1.44 volts at 15° C. One volt will be generated in a conductor which is moved across a magnetic field so as to cut the lines of force at the rate of 108 per second. Resistance to the flow of a (d) The Unit of Resistance. current is of the nature of an opposing force. The unit of resistance is such that one volt of E.M.F. will cause current to flow through it at the rate of one ampere per second. The practical unit is chosen equal to 109 C.G.S. units of force and is called the ohm. The adopted international ohm Is based upon the ohm equal to units of resistance of the C.G.S. system of electromagnetic units, and is represented sufficiently well for practical use by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 grams in mass, of a constant cross sectional area, and of the length 106.3 centimeters. For practical standards, coils of German silver or platinoid wire are wound on spools, standarized and placed in a suitable box, the coils being connected to brass bars on the top, so that by using plugs any resistance within the capacity of the box can be obtained. The megohm, or 1,000,000 ohms, is used in the measurement of high resistances, such as the resistance of insulators. For very small resistances the microhm, or roooooo ohm is used. (e) The Unit of Capacity. If one coulomb of electricity be stored in a recipient, for instance, in a coil of insulated wire, or in a system of flat parallel conductors, adjacent ones insulated from each other, called a condenser, and if this quantity tends to escape with an E.M.F. of one volt, the capacity of the recipient or condenser is unity. This unit is the farad. Expressed in another way, it is the capacity such that one volt will store in it one coulomb of electricity. For all ordinary capacities the microfarad, or 100 of the farad is used, the farad being very large, 10-9 C.G.S. units. 636 1000000 The capacity of the earth is 1083800 farad, or 636 microfarads. A Leyden jar with a total tinfoil surface of I square meter, and glass 1 millimeter in thickness, has a capacity of microfarad. (f) The Unit of Power. called the watt, and is the energy required to move one ampere per second through one ohm resistance. In other words, one volt of E.M.F. delivering one ampere of current per second represents a power of one watt. The watt is equal to 107 ergs per second of C.G.S. units. The horsepower is 33,000 foot-pounds of mechanical work per minute. The watt is 7 of the horse-power, and 1 H.P. = 746 The unit of electrical power is watts. (g) The Unit of Work. The unit of work is done when one watt of energy is expended per second, and is called the joule, which is also equal to 10' ergs. Joules of work are obtained by multiplying watts of power, or energy, by seconds of time. (h) The Unit of Induction. The induction is unity when the E.M.F. induced is one international volt, while the inducing current varies at the rate of one international ampere per second. This unit is called the henry, and is 109 C.G.S units. These international electrical units were legalized by Act of Congress, approved by the President, July 12, 1894, so that they take their place with other standards of weights and measures. II. RELATION OF QUANTITIES. 7. Ohm's Law. The simplest perhaps, and yet the most important relation of electrical or magnetic quantities to each other is that expressed in Ohm's law. This law expresses the relation of E.M.F., current, and resistance. Putting for current, E for E.M.F., and R for resistance, the law is expressed by the formula, Hence, current is obtained by dividing electromotive force by resistance. From the above formula, by simple transposition, or by the rules of simple division, we obtain E = IR, and R = E In words, these mean that the E.M.F. is equal to the product of current and resistance, and that resistance is equal to the quotient of E.M.F. by current. EXAMPLE. How much current in amperes will flow through an incandescent lamp whose resistance is 200 ohms, when an E.M.F. of 110 volts is applied to it? SOLUTION. I= E IIO R 200 =0.55 ampere. EXAMPLE. A battery has a resistance r of 3 ohms; what is its E.M.F. if it causes a current of 0.05 amperes to flow through an external resistance equal to 60 ohms? EXAMPLE. Find the resistance of an arc street lamp when a voltage of 50 is required to send 7 amperes through it. SOLUTION. R E 50 I = — = =7 ohms. Expressed in C.G.S. units, E = 103, R = 10o, and I = E 108 = R 109 = 101or. That is, the ampere is the of the C.G.S. electromagnetic unit of current. 8. Quantity, Electromotive Force and Capacity. — EXAMPLE. - How many coulombs of electricity will-flow through an incandescent lamp whose resistance is 200 ohms, when an E.M.F. of 110 volts is applied to it for 2 seconds? SOLUTION. EXAMPLE. E IIO Q=Ixt= Xt= X 2 = 1.1 coulombs. 200 A glass plate has a square of tinfoil pasted on each side, thus forming a storehouse or condenser. A battery whose E.M.F. is 10 volts is connected to both sides by wires, and thus gives it a charge of 0.000000001 coulomb = 0.001 microcoulomb: what is the capacity of the receptacle? SOLUTION. From obvious considerations the quantity stored will depend on the pressure applied and the capacity of the storehouse: that is |