In the course of these pages, reference has several times been made to the indicator. This instrument is of great service to the steam engineer in helping him to determine with accuracy questions of steam distribution, that, without its aid, would remain in the realms of speculation. Its diagram presents a reliable record of what is going on inside the cylinders of a steam engine; and master mechanics who desire to run their engines economically, can not afford to dispense with the accurate information imparted by the indicator respecting individual locomotives.
DESCRIPTION OF INSTRUMENT.
The indicator consists essentially of a small steam cylinder, whose under side is connected by pipes to the main cylinder of the engine under inspection. Inside the indicator cylinder is a nicely fitting piston, whose upward movement is resisted by a spring of known strength. The piston-rod passes up through the top of the indicator cylinder; and its extremity is connected with mechanism for operating a pencil, and marking on a card a diagram whose lines coincide with the movement of the indicator piston.
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.