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| Figure 1-2 Hydraulic (oil or water jack) analogy of a transformer. |

| distances traveled correspond to the amperes. The total power on both sides, however, is the same. Thus, if the power input on one side was 1 ampere at a pressure or voltage of 100 volts, the same power would be obtained on the other side, but this time 10 amperes would be received at a voltage of 10 volts. It will be noticed that the product of the volts and amperes is the same for both the input and output. This product then is a means of measuring the capacity of the transformer, and is termed volt-ampere (VA). This unit is usually too small for practical purposes and so the unit called kilovoltampere (kVA) is used. The prefix kilo means 1000, so that 1 kVA equals 1000 VA (also a kilovolt is equal to 1000 volts). Power transformers range in capacity from ½ kVA to about 5000 kVA for supply to distribution customers, and from about 100 kVA to some 500,000 kVA for bulk or wholesale power supply. |

| Referring to the hydraulic jack again, if a continuous motion were substituted for the up and down pressure applied to the piston of one of the cylinders, power would be transmitted only during that time that the piston moved to its ultimate position. There would be no way to have the piston move back so that it would be able to transmit power on another stroke. The same effect is true in the transformer. Hence, it will be seen that the transformer will not work for direct or continuous currents. |

| The transformer is a highly efficient device so that practically the full electrical power received from one circuit is delivered to the other. Since electric power (P) may be considered the product of voltage (E) and current (I) (that is, P = E × I), it is evident that the values of current in a transformer change in opposite fashion to those of the voltage. That is, in a step-up transformer where the voltage delivered is higher than that received, the current delivered is lower than that received; in a step-down transformer, where the voltage delivered is lower than that received, the current delivered is higher than that received. |

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