American Thompson Steam Gauge

The Thompson Improved Steam Indicator comes in a dark stained wooden box   285mm wide, 207mm deep, and 240mm tall.  It opens about halfway up its height to reveal a number of objects.  The box, as well as various components, are  serial numbered  4994.  Inside the box there are  pockets as well as mounts for the various objects to go into  which can be seen in the images provided.

In the center is the steam indicator, which resembles two off-center attached cylinders screwed onto a mount.  The cylinders are both  90mm in height and  50mm in diameter.  The top cylinder  has a slot to affix a cardstock, a stylus arm and pull string.  The bottom cylinder has the arm holding the stylus  and a spring inside to control the stylus arm.  The bottom of the bottom cylinder has threads and ties to the mount on the bottom of the box, as well as thumb bars on the bottom to attach or unattach it by hand.

The steam indicator comes with 6 springs numbered 10, 20, 30, 40, 60, and 80, which range in length from 53-63mm long.  These springs are threaded onto mounts on the upper half of the box and can replace the spring inside the bottom cylinder of the steam indicator. This allows the system to make more precise graphs depending on the pressure that the engine is putting out.

In addition to the above, there are also two globe shut-off valves with thread sizes of 20 and 25mm.  The valves come with caps for one side to prevent damage to the threads that could result in inadequate attachment . There is also a torque wrench used to tighten the lower cylinder onto the engine  that the indicator will be attached to. There is also a small vial of oil,  for the joints of the stylus arm or the springs. There was also a specialized wrench designed to tighten  the components in order to prevent gas leaks. The final component was a ruler with a screw driver end that allowed for the dismantling of the indicator as well as measuring the height of the graphs drawn by the arm allowing the user to find the pressure in the system.

Functional Description:

In order to use the Thompson steam indicator, the user fastened the stainless-steel ball valve to the steam engine being analyzed. The indicator was then be fastened to the open end of the ball valve via a threaded connection. With the ball valve opened, the steam within the engine’s piston exerts pressure on a spring enclosed within a 100 mm tall, 30 mm diam. cylinder of the indicator. A plunger connected to the spring is  forced upward depending upon the force received by the spring. The motion of the plunger moves the 80 mm long arm vertically, which determines the markings made by the attached pencil. The pencil marks a piece of paper, which is wrapped around the upper cylinder (90 mm tall and 51 mm dia) of the indicator. Wrapped around this upper cylinder is a string whose function is to rotate the cylinder about its center. If the string were left free to hang, the steam pressure pushes the pencil upward, making a straight vertical line on the paper. However, the string was fastened to a component of the engine to allow for the engine piston and the cylinder to move in unison. The string moves the cylinder of the steam indicator in unison with the piston throughout the entire stroke, and a continuous marking is made to illustrate the pressure at each point of the stroke as the pencil is moved upward and downward while the cylinder pivots throughout the process. Thus, the vertical motion of the pencil graphs changes in pressure, while the rotation of the cylinder indicates the phase of the piston cycle, and so in unison the pencil plots a diagram of the pressure cycle in the engine.

The indicator was designed to record the stroke of a steam engine, particularly locomotives. At the start of the stroke, the inlet valve opens completely, allowing the maximum force to be applied to the spring by the steam. The pencil marks a horizontal line at its highest point in the cycle from left to right. At cut-off, the inlet valve closes and expansion of the steam occurs, during which the force gradually decreases. This phase is indicated by a pencil mark resembling an exponential decay curve. At release, the exhaust valve opens, releasing the steam and the force is at its minimum, resulting in a straight horizontal pencil mark at the lowest point of the curve—this time from right to left. When the exhaust valve closes, compression occurs, causing a gradual rise in pressure. When the inlet valve opens again, the gradual increase in pressure becomes a sharp increase, causing the pencil to mark a straight vertical line returning to the first point of the cycle when the steam pressure exerts its greatest force on the spring. The end product of this device is a card illustrating the change in pressure throughout the continuous cycle of the steam engine. For an engine operating at 250 RPM, the device could generate one cycle plot (or card) per minute.

The spring within the indicator can be replaced with a larger spring to record higher steam engine pressures. The set of springs are marked according to their compressive strength: the spring marked #100 converts the force from 100 psi gauge-pressuresteam into a pencil movement of one inch, a #80 spring converts 80 psig steam into a movement of one inch, and so on.

Though the steam indicator was primarily used on locomotives, it could also be applied to traction engines and artillery. Due to the high cost of the indicator, it was seldom used on traction engines beyond the confines of the factory in which it was produced. When applied to heavy artillery, the oil in the recoil chamber would replace the steam as the working fluid and exert a pressure on the spring. The string would be fastened to the barrel to allow for the recoil movement to pull on the string.

The object was made by the firm the American Steam Gauge Company of Boston, MA. The indicator was initially invented by James Watt, who also is credited with the invention of the steam engine. Joseph Thompson is the creator of this steam engine indicator with his patent being granted on Aug 31, 1875. The instrument was manufactured by The American Steam Gauge Company, which was founded in 1851. The instrument was sold as the “American Thompson Improved Indicator”, and was considered to be high end among steam indicators of the time. The improvements of the Thompson indicator over previous models were that the writing component followed an elliptical pattern to maintain a straight line on the recording barrel. This resulted in less inertia and increased the engine speeds, and pressures that the indicator could be used at. The patent granted in 1904 to Earl Vaughn had the benefits of making the indicator easier to disassemble for adaptability of the indicator. For this reason it is believed that this indicator was manufactured sometime after 1904.

"The Story of the Steam Engine Indicator." last modified July/August 2001 http://www.farmcollector.com/steam-traction/story-steam-engine-indicator

Engineer: with which is incorporated steam engineering, volume 35 .. Vol. 42. (Chicago: The Engineer Publishing Company, 1905). 

Dorn, Harold. Dictionary of Scientific Biography. Vol. 14. (New York: Charles Scribner's Sons, 1976), 196-99.