The free-end travel and force of the C-tube are insufficient to operate many motion balance devices. Motion balance describes the means by which the motion of the pressure element moves the sector, mechanically transmitting a rotary motion to the pinion assembly and indicator. The spiral element can be considered to function as a series of C-tubes connected end to end. When pressure is applied, the element tends to uncoil and, by producing greater movement without amplification, yields greater accuracy and sensitivity.Less hysteresis and friction result because of the reduced need for addi-tional links and levers. In general, the same types of materials are used for spiral element construction as for C-tubes. Spiral pressure transducers produce more tip travel and higher torque capability at the free end than C-tubes. They are thus more capable of use as direct-drive readout devices. Instruments that utilize these types of transducers can respond to lower pressure and thus are more sensitive, partly because of the reduced friction involved in the movement.The operating range of spiral and helical elements is from a few inches of water to a few hundred pounds per square inch. These transducers are used primarily in pressure transmitters, pneumatic controllers and receiver instruments. Their use in pressure gages is limited.