Patent GB-483547-A (GB)
Title: Apparatus for investigating temporarily variable electrical fields
IPC: A61B5/04
Filed: 1936-08-18
Abstract: 483,547. Electrocardiographs. KINGS PATENT AGENCY, Ltd. (Heller, S.) Aug. 18, 1936, No. 22717. [Class 37] An electrocardiograph comprises three or more electrodes applied to the body at points other than along a straight line, so that two differences of potential are obtained between two pairs of points, these differences of potential being applied through an amplifier to two sets of deflecting devices of an oscillograph, the indicator of which is capable of two-dimensional motion, whereby there is traced out a polar curve which indicates at any instant the direction and magnitude of the field of the heart. Two pairs of electrodes 8, 9 and 10, 11 may be arranged on the patient's body, the angle between the two pairs of electrodes being made equal to that between the beam deflecting plates of the cathode-ray oscillograph used as recording instrument. The electrodes are connected through amplifiers 14, 15 to the deflecting plates of the oscillograph 21, so that the electron beam is deflected from its normal position in a direction parallel to that of the component of the electric field in the plane of the electrodes and to an extent which is proportional to the magnitude of the component. The fluorescent spot thus traces out a polar diagram showing the changes in magnitude and direction of the component in question, and this diagram may be recorded photographically. Three electrodes only may be used. When four electrodes are used, two of them may be employed first in one position and then in a second position displaced from but parallel to the first so that by making successive exposures two vector diagrams are obtained which can be observed stereoscopically. Alternatively, two sets of apparatus giving simultaneous diagrams may be used. In order to give a picture of uniform intensity irrespective of the speed of movement of the fluorescent spot, the intensity of the beam is controlled by applying, between the Wehnelt cylinder and the filament of the cathode ray tube, a voltage the magnitude of which is proportional to the rate of change of the deflecting voltages. For this purpose, a transformer winding is connected between the terminals of resistances 37, 38 connected in the anode circuit of the last stage of the amplifiers 14, 15, the secondary winding of which is centretapped and is connected to a full-wave rectifier. The voltage variations across a resistance connected between the centre-tapping of the transformer and the rectifier filament controls the intensity of the cathode ray beam. The intensity of the beam may when controlled in this manner be employed to represent the course in space of the voltage vector of the field by taking off from the body a third component perpendicularly to the plane containing the electrodes 8 ... 11 and using this voltage to control the beam intensity. The speed and direction of rotation of the vector at all points on the diagram may be indicated by periodically varying the intensity of the record-making light beam by suddenly increasing or decreasing the intensity and then permitting a gradual change to normal. Such an arrangement may be obtained by means of a rotating disc 51 through which the light passes on its way to the photographic screen 28. The disc is transparent except for the provision of black wedge-shaped portions 52 which cause the required changes in light intensity. Alternatively, the intensity of the electron beam may be varied in the same way by means of a controlling voltage applied to the focusing cylinder of the tube and derived from a condenser which is periodically charged from a battery and discharged by means of a discharge tube connected across it. Alternatively, this control voltage may act on a Kerr cell arranged in the path of the light. The sound of the heart may also be studied at the same time as the electric field produced by its movement is studied, by causing the sound to control through a microphone the intensity of the light spot producing the picture. This control is made alternative to the control for indicating the direction and speed of change of the vector by means of a switch. The scale of the diagram may be increased at certain points to render the diagram more easily understood by varying the degree of amplification as desired.