OPERATION OF THE INDICATOR.
Fig. 30.
Fig. 31.
[Fig. 30] is a perspective view of the Tabor indicator, an instrument well adapted for application to high-speed engines, such as the locomotive. In [Fig. 31] the indicator is shown in section. The construction of the instrument can be well understood from a careful examination of these figures. In the sectional figure, the piston is seen with its rod encircled by the spring passing up through the top of the cylinder, and connecting with the lever which operates the marking-pencil. By ingeniously designed mechanism, the lever is made to move the pencil in a perfectly straight vertical line. The card to be marked is fastened in the paper drum attached to the indicator. This drum receives a circular motion from a cord which is operated by the cross-head, and this connection is so arranged that the drum will begin to move round just as the main piston begins its stroke. The circular motion of the drum is continued till the piston reaches the end of its stroke, when the drum reverses its movement, and returns to the exact point from whence it started. Now the indicator cylinder being in communication with the main cylinder, when the latter begins to take steam, the pressure will be applied to the indicator piston, which is pushed upward, at the same time transmitting its movement to the pencil. The indicator piston will rise and fall in accordance with the steam-pressure in the cylinder; and, the circular movement of the drum coinciding with the cross-head movement, the pencil will describe a diagram which represents the pressure inside the main cylinder at the various points of the stroke.