CATALOG OF THE MECHANICAL COLLECTIONS 93 



and a peculiar parallel motion for effecting the straight line motion 

 of the marker in which "the lever is connected with the piston-rod 

 by a joint, and not indirectly by a link, as in the Kichards indicator." 



THOMPSON INDICATOR 



U.S.N.M. uo. 309644 ; original ; gift of N. C. Hunt ; not illustrated. 



This indicator was made by the American Steam Gauge Co., of 

 Boston. It is marked "J. W. Thompson Pat. August 31, '75 Pat. 

 June 26, 1883, No. 4302." 



In this indicator the piston rod is hollow and serves only as a 

 guide for the piston. The pencil mechanism is connected to the 

 piston by a very light rod that passes through the piston rod and 

 is attached to the piston with a swivel joint. This permits the 

 connecting rod to swing through a slight arc, which in turn permits 

 the use of a very simple and light parallel motion. 



The piston is a light cylindrical shell provided with three grooves 

 that collect moisture and steam to lubricate and seal the piston. The 

 inner wall of the cylinder is a liner separate from and secure*! to 

 the inclosing cylinder only at one end so that it is free to expand 

 and contract with temperature changes, thus avoiding distortion. 



INDICATOR WITH REDUCING WHEEL, 1930 



U.S.N.M. no. 309833; original; gift of the Crosby Steam Gage & Valve Co.; 

 not Illustrated. 



This indicator, designed to meet the requirements of modern high- 

 speed engines, employs the lightest construction consistent with 

 strength and accuracy. It is equipped with a reducing wheel, which 

 is a self-contained device capable of reducing engine strokes of 14 

 to 72 inches to the proper stroke of the paper drum. 



Tlie cylinder of this indicator is supported so that its lower end 

 is free and its longitudinal expansion or contraction is unimpeded. 

 The annular space between the cylinder and the casing is designed to 

 serve as a steam jacket. The piston is an extremely thin steel shell 

 with shallow channels on its outer surface to provide steam packing 

 and moisture lubrication. The piston rod is hollow and is connected 

 to the pencil mechanism by means of a swivel head that can bo 

 screwed in or out of the rod to adjust the position of the diagram 

 on the paper. The pencil mechanism is kinematically a pantograph 

 that theoretically gives the pencil point a movement exactly parallel 

 to the piston and the amount of the movement of the piston is multi- 

 plied six times at the pencil point. The design of the piston spring 

 is peculiar to this indicator. It is made of a single piece of spring 

 steel wire wound from the middle into a double coil, the ends of 

 which are screwed into a metal head drilled helically to receive the 

 spring. 



