[4] NOTE.—The author adheres to the term virtual because in mechanics the adjective effective is used to denote the difference of two opposing forces; for instance, at any instant in the operation of a steam engine, effective pressure = forward pressure - back pressure, hence, to be consistent in nomenclature, the term effective cannot be used for the forward or virtual pressure, that is, the pressure impressed on an electric circuit.
In the operation of a steam engine, there are two pressures acting on the piston:
- 1. The forward pressure;
- 2. The back pressure.
The forward pressure on one side of the piston is that due to the live steam from the boiler, and the back pressure, on the other side, that due to the resistance or opposition encountered by the steam as it exhausts from the cylinder.
In order that the engine may run and do external work, it is evident that the forward pressure must be greater than the back pressure, and it follows that the pressure available to run the engine is the difference between these two pressures, this pressure difference being known as the effective pressure, that is to say
effective pressure = forward pressure - back pressure
Thus, electrically speaking, the effective voltage is that voltage which is available for driving electricity around the circuit, that is,
| effective volts | = | virtual volts - back volts |
| = | virtual pressure - (virtual pressure - drop) |
In the case of the steam engine, the forward pressure absolute, that is, measured from a perfect vacuum is the virtual pressure (not considering the source). The back pressure may vary widely for different conditions of operation as illustrated in figs. 2,493 and 2,494.
