Patent GB-688235-A (GB)
Title: Electrical recording instrument, particularly for an electrocardiograph
IPC: G01R13/04
Filed: 1951-02-27
Abstract: 688,235. Recording apparatus. CAMBRIDGE INSTRUMENT CO., Inc. Feb. 27, 1951 [March 23, 1950], No. 4779/51. Class 106 (iv). [Also in Groups VI and XXXVI] An electrical recording instrument comprises a galvanometer unit having a coil which is rotated about a predetermined axis, a stylus assembly attached to the coil and adapted to move along an arcuate path when the coil rotates, the stylus assembly consisting of a stylus element adapted to be electrically heated and a boom secured to the stylus element and to the coil and including electrical circuit connections for the stylus element; a platen assembly including a concave recordsupporting surface arranged to form a record chart into the shape of a semi-cylindrical surface having its axis substantially co-incident with the predetermined axis of rotation of the coil and With the concave recording surface extending along the arcuate path of the stylus and spring mounting means for the galvanometer coil. In the embodiment described an electro-cardiograph comprises a galvanometer coil 6, Fig. 3, carrying a boom 8 which extends diagonally downwardly and carries a stylus 10 at its lower end for recording on a strip of paper 4 which is drawn from a supply roll 2 through a resiliently mounted platen assembly 16 by a feed roller 12 against which it is held by a pressure roller 14. A lower platen 18 has a concave surface 20 forming part of a cylinder whose axis coincides w.th the axis of rotation of the coil 6 and as it is drawn along the paper strip 4 is caused to conform to this curvature by an arcuate lip 24 on an upper platen 19 which is mounted by resilient brackets 21 on the lower platen 18 for limited upward movement, restricted by screws 25, to enable a new sheet to be threaded between the platens. The paper is such that it does not buckle or wrinkle when diverted from its normal flat form to the curved form, it has a coated upper surface which is white and ruled with longitudinal and transverse reference lines and a black line is produced thereon by heating the stylus, the coating acting as a lubricant. The platens are of plastic and a strip of heat conducting metal 26, e.g. aluminium is embedded in the platen 18 beneath the lip 24 and lower than the surface 20 so that if the paper supply is exhausted the stylus contacts the strip which prevents overheating by conducting heat away. The coil 6 is supported by four beryllium copper springs 34, 36, 38, 40, Fig. 1, which tend to hold it horizontal but permit rotation about a horizontal axis, the sides of the coil moving in gaps between poles 30 of a permanent magnet 28 and a cylindrical core 32 which has a mounting 46 formed by a central spring 48 urging apart two cylinders 50 with reduced ends 52 entering recesses in the ends of the coil 6 and clamped by set screws 54. The coil 6 is deflected by signal currents passed through it through springs 34, 36, while heating current is passed through the boom 8 and stylus 10 through springs 38, 40. A metal shield 27 clamped to the upper platen 19 shields the boom from the magnet 28 and facilitates lifting the platen 19 when inserting a new roll of paper. The galvanometer unit is mounted from its top on a rigid metal case 70 which is hinged to the base plate 3 by a hinge 72 and rigidly held in closed position by a clamp 74 having a cam action. When the clamp is released the case 70 may be swung about the hinge 72 moving the galvanometer, boom and stylus free from the platen assembly to facilitate the renewal of the paper. By releasing a clamping screw 78 the hinge 72 is freed and the whole unit may be removed for service or replacement. The platen assembly 16 is aligned with the axis of the coil 6 by lugs formed in the sides of the case 70 engaging anvil recesses in the lower platen 18 and pressing it down to a predetermined position relative to the axis when the clamp 74 is engaged. The boom 8 is formed of two beryllium copper tubular struts 56, 58 and three reinforcing cross-pieces 60, 62, 64 of glass fibre cemented with silicone cement and the stylus 10 is a U-shaped element of electrical resistance wire with its ends soldered into the ends of the struts 56, 58. The boom with the stylus attached is given a thin gold plating to protect the soldered joints.