Abstract
The principle of the electro-magnetic flow meter 2 is based on the fact that an electromotive force is induced in the blood as it flows through a magnetic field (the direction of flow forming preferably a right angle with the magnetic lines of force).∗ The induced e.m.f. is linearly proportional to the velocity of flow. It is tapped by two electrodes placed at right angles to the magnetic lines of force and to the direction of flow. Owing to the electrical conductivity of the blood vessel wall the pick-up electrodes merely have to touch the outside of the vessel to establish a sufficient electrical contact with moving blood. Fig. 1 shows the perspective view of the arrangement and the dimensions of the magnet.
The modification presented here employs an electro-magnet fed by an alternating current. The use of an alternating magnetic field eliminates the disturbances due to polarization and variable galvanic potentials and makes it possible to use an a.c. amplifier. It introduces, however, the new difficulty of eliminating the a.c. voltage induced in the lead wires at. zero flow. This interference has been abolished by the use of a special sleeve S and a compensating potentiometer P in conjunction with a filter F in the amplifying circuit. (Fig. 2.) The bakelite sleeve, S, contains a slit, S1, which allows a blood vessel to slip into the round channel of the sleeve. This establishes the proper contact between the vessel wall and the electrodes, E1 and E2 (2/56 brass screws screwed into the bakelite flush with the channel wall). A coil, C, of 3 turns of copper wire is wound around the sleeve imbedded in the groove, G.
Get full access to this article
View all access options for this article.