is movable within the accumulator 127 a in the embodiment shown in fig4 one conductive strip 407 is disposed along the length of each rubbing strip 405 within the accumulator 127 a as illustrated in fig6 the conductive strips 407 are embedded in or otherwise attached to the rubbing strips 405 each conductive strip 407 extends beyond the profile of its associated rubbing strip 405 so as to be capable of coming into contact with the conductive surface area s of the piston 401 as the piston 401 travels within the accumulator 127 a in another embodiment the conductive strips 407 can be placed on top of the rubbing strips 405 rather than being embedded in the rubbing strips 405 one end of each conductive strip 407 terminates for example at an end cap 408 of the accumulator 127 a the end cap 408 includes typical openings and porting for communicating fluids e g gas and or liquid to the accumulator 127 a which do not constitute part of the invention and are therefore not shown or described in detail the other end of each conductive strip 407 is connected to a power source 411 the conductive strip 407 connects to the voltage current source through a connector such as a bulkhead connector not shown when the conductive surface area of the piston 401 is in contact with the conductive strips 407 a circuit is formed with electrical characteristics e g voltage current resistance that vary as the piston moves along the length of the accumulator 127 a the length of the circuit formed between the piston 401 and conductive strips 407 decreases as the piston 401 moves through the interior of the accumulator 127 a toward the power source.