A Hall effect transducer for measuring length changes in mammalian diaphragm

When a conductor through which a current is flowing is brought into a magnetic field, the isopotential lines are distorted (E. H. Hall, 1879). When a constant current is made to flow through a Hall generator, the measured voltage perpendicular to the current flow can be made to vary by altering the distance between a permanent magnet and a Hall generator. A length transducer useful for measuring length changes in the diaphragm of a dog is made by connecting a commercially available Hall generator to a Plexiglas plate (5 X 10 X 3 mm, 0.5 g), and suturing the plate to the surface of the muscle. A permanent magnet (20 X 10 X 4 mm, 1.5 g) is attached to the diaphragm at a distance of 2 or 3 cm along the expected direction of length change. Fine wires from the terminals of the Hall generator are connected to an external constant-current source and to solid-state circuitry which provides amplification, linearization, and temperature compensation of the output signal. Model experiments indicate that the length measurements may be accurate within 5% if the distance between the magnet and Hall generator is greater than 5 mm. With this type of transducer measurements of segmental length changes in the diaphragm of an anesthetized dog have been successfully recorded.