Wires and other electrical conductors do not offer a perfectly free path to an electric current, but also possess a resistance. It is the potential of the electro-motive force which overcomes the resistance and pushes the current through the wire. Advantage has been taken of the fact to fix a unit of electrical pressure called the _volt_. The pressure of the water in a water-pipe is measured in pounds, but the pressure of an electric current in a wire is measured by _volts_. The volt is the unit of electrical force which will cause a current of one ampere to flow through a resistance of one _ohm_. The Ohm The ohm is the unit of electrical resistance. The standard ohm is the resistance offered by a column of pure mercury having a section of one square millimeter and a length of 106.28 centimeters at a temperature of 0° centigrade. The pressure which will force sufficient current through such a column of mercury to deposit 1.177 grammes of copper in one hour is a volt, and in doing so has passed a current of one ampere through a resistance of one ohm. The units ohm, ampere, and volt, were named in honor of the three great electricians: Ohm, Ampère, and Volta. These three units bear a very close relation to each other which is explained by Ohm’s Law. Ohm’s Law is a simple statement of facts which it is well for the young electrician thoroughly to understand, for it might almost be said to be the basis of design of almost all electrical instruments. It is simply this: The strength of a current equals the voltage divided by the resistance. It may be expressed in symbols by: _C = E/R_. Where C is the current in amperes, E is the potential in volts, and R the resistance in ohms. By way of a simple example, we will suppose that a small telegraph sounder is connected to a battery and that the voltage of the battery is _ten volts_. We will further suppose that the resistance of the sounder connecting wires and the battery itself is _five ohms_. Knowing these two facts, it is very easy to find out how many amperes are flowing through the sounder by substituting these values in the equation as follows: C = E/R E = 10 volts and R = 5 ohms therefore C = 10/5 or 2 amperes In order to indicate fractions or very large values of the ampere, volt, and ohm, it is customary to use the following terms: Milli-volt = 1/1000 of a volt Mill-ampere = 1/1000 of an ampere Kilo-volt = 1000 volts Meg-ohm = 1,000,000 ohms The Watt It is no doubt perfectly plain that the water in a certain size of pipe at a pressure of 100 lbs. is more powerful than a stream of water in the same size of pipe at 25 lbs. pressure. Likewise a current of electricity represents more power at 100 volts potential than the same current would at 25 volts. The unit of electrical power is called the _watt_. A watt is represented by a current of one ampere flowing through a wire at a potential of one volt.