To do this all that is necessary is to take out the piston and spring, draw the atmospheric line, and allow the drum to return to its original position; then lift pencil. The line drawn should be a horizontal straight line.

When using an indicator for taking diagrams from a gas engine very great difficulty is experienced, owing to the sudden rise of pressure, and abnormally high cards are often obtained, in some cases reaching 400 to 600 lbs. above atmosphere. The strain is invariably severe the temperature may affect the spring, and there is considerable risk of the piston sticking; therefore it is not desirable to place too much reliance upon the indicated horse-power of a gas engine.

Diagrams taken when the engine is cold are very different from those when the engine is hot. Then, again, the first diagram after a cut-off-after the cylinder and combustion chamber have been cleared of the products of combustion-is invariably larger than the succeeding ones.

Thompson's Indicator

Fig. 175 is a part sectional elevation of a small Thompson indicator, as made by Messrs. Schäffer & Budenberg, the chief distinguishing feature of which is the novel arrangement of light levers carrying the pencil.

The parallel motion is carefully designed to ensure that the pencil point describes a straight line, and the motion of the pencil point should be precisely proportional to that of the indicator piston throughout the stroke.

The link connecting the pencil arm to the piston rod has a ball-and-socket joint at the loose end to allow free motion.

A movable collar around the upper part of the cylinder swings round against a stop to prevent undue pressure of the pencil on the paper. This collar carries a fixed standard with radius bar, as well as a link with the fulcrum of the pencil arm at its upper end.

The parts which move with a high velocity are made as light as possible, consistent with strength and stiffness, in order to reduce their inertia to a minimum.

The paper drum is shown separately in section, fig. 175.

By removing the outer cylinder, loosening the nut on the top of the drum spindle, and by turning the disc holding the flat spring inside, the latter may be readily adjusted to suit any speed of engine up to 400 revolutions per minute.

A

FIG. 175. THOMPSON INDICATOR

When using this indicator for taking diagrams with exceptionally high maximum pressures a special piston A, one-half the area of the ordinary piston, is used, thus reducing the stress on the levers and motion bar and effectually overcoming the wave oscillation.

The Crosby Indicator

Fig. 176 is a sectional elevation of the Crosby indicator. It has a very light piston, parallel motion, and drum. The spring is made of one piece of steel wire, wound from the middle in a double spiral, its ends being screwed into the wings of the spring head D fixed to the cylinder cap A.

K

FIG. 176. CROSBY INDICATOR

springs is made by simply On the middle of the spring

The final adjustment of the screwing in or out of the head D. is brazed a small steel ball, which fits a socket in the hole on the piston rod, and is adjusted to the proper lightness by the screw G, so as to allow free motion and prevent sticking or twisting, the thrust on the spring producing a perfectly axial compression.

The steam or gas from the engine passes into the cylinder F and H around the piston, and keeps it from being forced sideways against the cylinder. It is desirable not to have the diagram more than 175 ins. high; hence, when measuring high pressures, use stiff piston-springs, which will at the same time lessen vibrations.

The very slight movement of the indicator-spring is then magnified to six times the range by the pencil.

The long pencil arm, necessary for this multiplication, is the weak part of this indicator when used for gas-engine purposes.

In careless hands the friction of the pencil on the paper sometimes amounts to tearing of the paper, or bending of the pencil when it is pressed too hard against the paper. This is prevented in the Crosby by a stop for the sleeve on the cylinder carrying the pencil movement. A screw comes against the stop, and can be adjusted to a nicety, so that the pencil point is allowed just to touch the paper and make a visible mark.

The paper drum has a short spiral spring, which can be tightened and adjusted by the collar and lock-nut at the top to suit the speed of the engine to be indicated. This spring offers increasing resistance, as it is compressed by the rotation of the drum, overcomes the inertia and friction of the drum, and thus maintains a constant stress on the cord connected by the reducing-gear to the engine-piston. With strengthened pencilarm, and when used in a careful manner by practical and intelligent engineers, the Crosby instrument gives good and reliable cards. The Crosby Co. are now making a special gas-engine indicator for use on engines with a high initial pressure. The pencil motions of this instrument will be stronger than the ordinary steam-engine pattern, and the piston is only one-half the diameter.

The Tabor Indicator

Fig. 177 is an external elevation and fig. 178 a sectional elevation of the Tabor Indicator.

Referring to figs. 177 and 178, it will be seen that the most noticeable peculiarity of this instrument is the means employed

N

to communicate a straight light movement to the pencil. A plate containing a curved slot is fixed in an upright position and secured to a swivel plate on the cover of the steam cylinder. This slot serves as a guide, and controls the motion of the pencil bar. A pin which carries a roller is fixed on one side of the pencil bar, and this roller is fitted so as to roll freely from end to end of the slot. The position of the slot is so adjusted,

and the pin attached at such a point on the pencil bar, that the curve of the slot compensates the tendency of the bar to move in a circular arc, and the end of the bar, which carries the pencil, moves up and down in a straight line when the roller is moved from one end of the slot to the other. It will be apparent that there is little chance for friction in this movement, and also the lightness of the bar and connections insures that