paddle wheel. Other circuit elements A diode is equivalent to a one-way check valve with a slightly leaky valve seat. As with a diode, a small pressure difference is needed before the valve opens. And like a diode, too much reverse bias can damage or destroy the valve assembly. A transistor is a valve in which a diaphragm, controlled by a low-current signal (either constant current for a BJT or constant pressure for a FET), moves a plunger which affects the current through another section of pipe. CMOS is a combination of two MOSFET transistors. As the input pressure changes, the pistons allow the output to connect to either zero or positive pressure. A memristor is a needle valve operated by a flow meter. As water flows through in the forward direction, the needle valve restricts flow more; as water flows the other direction, the needle valve opens further, providing less resistance. Practical application On the basis of this analogy Johan van Veen developed around 1937 a method to compute tidal currents with an electric analogue. After the North Sea flood of 1953 in The Netherlands he elaborated this idea, which eventually lead to the analog computer ‘’Deltar’’, which was used to make the hydraulic computations for the closures in the framework of the Delta Works. Principal equivalents EM wave speed (velocity of propagation) is equivalent to the speed of sound in water. When a light switch is flipped, the electric wave travels very quickly through the wires. Charge flow speed (drift velocity) is equivalent to the particle speed of water. The moving charges themselves move rather slowly. DC is equivalent to a constant flow of water in a circuit of pipes. Low frequency AC is equivalent to water oscillating back and forth in a pipe Higher-frequency AC and transmission lines is somewhat equivalent to sound being transmitted through the water pipes, though this does not properly mirror the cyclical reversal of alternating electric current. As described, the fluid flow conveys pressure fluctuations, but fluids do not reverse at high rates in hydraulic systems, which the above "low frequency" entry does accurately describe. A better concept (if sound waves are to be the phenomenon) is that of direct current with high-frequency "ripple" superimposed. Inductive spark used in induction coils is similar to water hammer, caused by the inertia of water Equation examples If the differential equations are equivalent in form,