In its modern form ([fig. 20]) the photographic plate is replaced by a sheet of sensitized paper wound round a drum which rotates at a known rate—say, once in 12 hours—by means of internal clockwork, shown to the left of the figure. The galvanometer is placed at the opposite end, and the mirror is illuminated by means of an electric lamp placed externally, the rays from which are reflected from a prism in the interior on to the mirror. The ray of light leaving the mirror is broken into two portions, one of which passes through a narrow slit on to the sensitized paper, whilst the other portion is reflected on to a ground-glass scale on the lid, divided so as to read temperatures. In this manner the arrangement serves not only as a recorder, but also indicates the existing temperature without necessitating the examination of the sensitized paper. The whole arrangement is made impervious to light, so that it may be used in daylight. A dark room is necessary for fixing the records. When desired, records of two or more pyrometers may be taken on the same sheet, a clockwork device being used to switch each instrument in turn on to the galvanometer for a given period, an external dial indicating which pyrometer is for the time being in circuit.
Whilst it is a drawback to the use of this recorder that the record is not visible, the use of a mirror galvanometer confers a high degree of sensitiveness to the instrument, not possessed by the recorders to be described subsequently.
Fig. 21.—Principle of Thread Recorder.
The Thread Recorder.—In this instrument an intermittent record is secured in ink, possessing the advantages of visibility during the period over which readings are taken, and of permanence without subsequent treatment of the chart. The principle is shown in [fig. 21], where A is a boom terminating in a V-shaped piece of ivory, and attached to the galvanometer suspension B. By means of a cam E, rotated by clockwork, a bar D is made to descend at stated intervals, pressing the end of A on to an inked thread G, and causing the thread to touch a paper wound round the drum C. This drum rotates on its axis once in 25 hours by the action of internal clockwork. The continued rotation of the cam E alternately raises and depresses the boom A, leaving it free for a sufficient time to enable it to attain the position it would occupy if the mechanism were absent. The thread G is passed over pulleys, and is wound round through an ink-well, so that the portion opposite A is always moist. With the bar D descending every two minutes, the successive dots form a nearly continuous line. The paper on C is divided horizontally into temperatures, and vertically into time units, so that the temperature existing at any given time may readily be ascertained. The front of the bar D, or a separate strip parallel to it, is divided so as to enable temperatures to be read without reference to the chart. The actual instrument is shown in [fig. 22]. When several simultaneous records are required, the drum C is extended, and other galvanometers introduced, to which the separate pyrometers are connected. Several records can be taken on one chart by introducing a clockwork mechanism to couple each pyrometer in turn to the one galvanometer.
Fig. 22.—Thread Recorder.