Ans. An ampere is the current produced by an E. M. F. of one volt in a circuit having a resistance of one ohm. It is that quantity of electricity which will deposit .005084 grain of copper per second. =Ques. What is an ohm?= Ans. An ohm is equal to the resistance offered to an unvarying electric current by a column of mercury at 32° Fahr., 14.4521 grams in mass, of a constant cross sectional area, and of the length of 106.3 centimeters. =Ohm’s Law.=--_In a given circuit, the amount of current in amperes is equal to the E. M. F. in volts divided by the resistance in ohms; that is:_ current = pressure / resistance = volts / ohms expressed as a formula: I = E/R (1) in which I = current strength in amperes; E = electromotive force in volts; R = resistance in ohms. From (1) is derived the following: E = IR (2) R = E/I (3) From (1) it is seen that the flow of the current is proportional to the voltage and inversely proportional to the resistance; the latter depends upon the material, length and diameter of the conductor. Since the current will always flow along the path of least resistance; it must be so guarded that there will be no leakage. Hence, to prevent leakage, wires are _insulated_, that is, covered by wrapping them with cotton or silk thread or other insulating material. If the insulation be not effective, the current may leak, and so return to the source without doing its work. This is known as a _short circuit_. The conductor which receives the current from the source is called the _lead_, and the one by which it flows back, the _return_. When wires are used for both lead and return, it is called a _metallic circuit_: when the ground is used for the return, it is called a _ground circuit_. An electric current is said to be: 1. _Direct_, when it is of unvarying direction; 2. _Alternating_, when it flows rapidly to and fro in opposite directions; 3. _Primary_, when it comes directly from the source; 4. _Secondary_, when the voltage and amperage of a primary current have been changed by an _induction coil_; 5. _Low tension_, when its voltage is low; 6. _High tension_, when its voltage is high. A high tension current is capable of forcing its way against considerable resistance, whereas, a low tension current must have its path made easy. =Production of the Electric Current.=--To produce a steady flow of water in a pipe two conditions are necessary. There must first be available a hydraulic pressure, or, as it is technically called, a “_head_” of water produced by a pump, or a difference of level or otherwise.