US912680A - Steam-engine and like indicator. - Google Patents

Description

B. HOPKINSON. STEAM ENGINE AND LIKE INDICATOR.

APPLICATION FILED FEB. 14, 1907.

6 SHEETS-SHEET 1 Patented Feb. 16, 1909.

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APPLICATION FILED PEB.14, 1907. 912,680, Patented Feb. 16, 1909.

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.B. HOPKINSON.

STEAM ENGINE AND LIKE INDICATOR. APPLIOATION FILED FEB. 14, 1907'.

91 2,680, Patented Feb. 16, 1909.

ATFURNEYS B. HOPK'INSON.

STEAM ENGINE AND LIKE INDICATOR. APPLICATION r1131) 213.14, 1907.

912,680. Patented'Feb. 16, 1909.

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B. HOPKINSON. STEAM ENGINE AND LIKE INDICATOR. APPLICATION FILED FEB. 14, 1907. v 91 2,680, Patented Feb. 16, 1909;

5 SHEETS-SHEET 6.

BERTRAM HOPKINSON, OF CAMBRIDGE, ENGLAND.

STEAi I-ENGINE AND LEKE INDICATOR.

Specification or Letters Patent.

Patented Feb. 16, 1909.

Application filed February 14-, 1907. Serial No. 357,508.

To all whom it may concern.

Be it known that i, BERTRAM Hornnvson, a subject of the King of Great Britain, residing at the Engineering Laboratory, Qambridge, in the county of Cambridge, England, engineer, have invented certain new and useful Improvements in Steam-Engine and Like Indicators, of which the following is a specification.

This invention relates to indicators for recording the pressure in reciprocating engines. Hitherto indicators for this purpose have been of two types. First, there is a well known steam engine indicator in Wnich a piston in a chamber communicating with the engine cylinder is caused to compress a spring. This movement of the piston is magnified by a system of levers and is transferred to a pencil which is pressed against a paper carried on a reciprocating drum. T he drum is connected to the piston of the engine and is thus caused to move at right angles to the motion of the pencil, to an extent proportionate to the motion of the engine piston. This type of indicator is not suitable for use with high speed engines on account of the inertia of. the moving parts. Moreover, f om the same cause it is unable to follow accurately the very rapid variation of pressure occurring at the moment of explosion in a gas engine, even at slow speeds. in the second type of indicator in common use the piston is replaced by a diaphragm which forms part of a wall of a chamber connected to the engine cylinder. The movement of the diaphragm produced by the varying pressure is transferred to a mirror which it causes to turn about one axis, while other mechanism connected to the piston or crank shaft of the engine causes the mirror to turn about a second aXis perpendicular to the first through an angle proportionate to the motion of the engine piston. A beam of light from a fixed source is reflected from the mirror and is brought to a focus on a ground glass screen. The point or spot of light so produced traces out a curve whose ordinates are proportionate to the motion of the diaphragm, while the abscissa; are proportionate to the motion of the engine piston. This second form of indicator is suitable for use with engines of high speed, since the inertia of its moving parts is small; but it'suffers from the disadvantage that the movement of the diaphragm and the corresponding movement of the mirror and the spot of light, are not proportionate to the change of pressure. The means hitherto used for producing that movement of the mirror and the spot of light, which corresponds to the motion of the engine piston, are inaccurate, delicate, and apt to get out of order. Moreover, diagrams procuced in this way cannot well be seen unless a very powerful source of light be used or the room be darkened, and it is difficult or impossible to make accurate measurements of the diagrams except by means of photography.

The chief object of my invention is to combine the advantages of both types of indicator, and to produce an apparatus which is simple to construct and easy to apply to the engine; and in which, at the same time, the moving parts are light and move through very small distances, so that the a )paratus can correctly respond to eXtremey rapid changes of pressure, and readily coo aerate with the means hereinafter described or the easy observation and measurement of the diagrams obtained. For this purpose I make use of a mirror which reflects a beam of light and which is tilted about two axes in its plane at right angles to one another as in the diaphragm type of indicator alluded to above. The movement about the first aXis is produced by the pressure acting upon a spring controlled piston and that about the second axis by the engine piston acting upon an oscillatory frame which is capable of movement about the axis of the spring controlled piston. The first mentioned move ment may be called the pressure motion, and the second the piston motion.

In order to observe and measure the optical diagram obtained, I cause the beam of light emanating from a fixed luminous point or source of light to be reflected from the mirror to a lens of considerable diameter and thence to reach the eye of the observer without falling u on a semi-opaque screen.

In order t at my said invention may be clearly understood and readily carried into effect I will describe the same with reference to the accompanying drawings, in which z- Figure l is a side elevation, Fi 2 a plan, and Fig. 3 a vertical section on the line 11 of Fig. 1, showing a convenient form of my improved apparatus. Figs. 4 and 5 are elevations taken at right angles to each other and show means that may be employed for imparting the piston motion to the apparatus. Fig. 6 is a side elevation and Fig. 7 a vertical section on the line 22 of Fig. 6 showing a modified form of the apparatus for enabling it to indicate pressure below as well as above that of the atmosphere. Fig. 8 is a vertical section showing a further modification in which the apparatus is provided with two mirrors. Fig. 9 shows diagrammatically one method of effecting observation and measurement of the optical diagrams obtained, Fig. 10 shows a modified method. Fig. 11 is a plan view and Fig. 12 an elevation showing diagrammatically the general arrangement of the means for optica lly observing the diagram obtained, and Fig. 13 is a front elevation of the transparent screen on fvhlilch the diagram is traced by the spot of Referring more particularly to Figs. 1 to 8, A is a metal block which is firmly screwed into the wall B of the engine cylinder. It is bored out along its axis, and in the bore (which communicates with the interior of the engine cylinder) a piston C is fitted to slide easily. The upper part of the block A is turned conical. Fitting accurately over this conical part is a frame D which carries a spring blade E, which is so placed that in its initial position it just'clears the upper end of the piston C. Carried on the frame D are two vertical pieces of spring G, G, and in holes at the upper ends of these a spindle H is pivoted. Firmly fixed to the spindle is the mirror I, its plane being roughly parallel to the axis of the block A. At right angles to the spindle and fixed to it is a short arm J (Figs. 2 and 3) and this is connected at its outer end to the middle point of the spring blade E, by means of a short piece of straight and fairly stifi spring K. The block D is held upon the conical seating of the block A by. means of a nut and lock-nut L, L, so that it can turn upon the cone bearing of the block A without shake. The piston movement is given to the mirror by causing the frame D with the attach ed mirror to oscillate about the axis of the block A in time with the engine piston and through an angle proportionate to the travel of the said piston. This oscillatory movement may be obtained by the means diagrammatically shown by Figs. 4 and 5 in which an eccentric N is fixed to the crank shaft M of the engine, the throw of the eccentric being relatively to its diameter in the ratio of the stroke of the engine to the length of its connecting rod. A lever O pivoted at P, carries at its lower end a roller Q, which is pressed by a spring or other convenient means against the eccentric. As the crank shaft rotates, the motion of any point R on the lever will then be a reduced copy of the motion of the engine piston. To transfer the motion to the indicator, a lever S clamped to the frame D is jointed to the lever O at the point R. The lever S and with it I i i the frame D then oscillate through an angflie proportionate to the motion of the point and therefore to the motion of the engine piston. in order to take up the slight curvature of the path of the point R, the lever S may conveniently consist of a flat piece of steel which will bend in a vertical but not in a horizontal plane, the joint at B being a ball and-socket joint to provide for the vertical motion.

The pressure movement, in the example shown, is obtained by the pressure of the piston C upon the spring E, in consequence of which the latter is deflected vertically through a small distance proportionate to the pressure in the engine. The end of the arm J is pushed upward by the spring K through a distance equal to the deflection of the spring E, and the spindle H accordingly turns through an angle proportionate to the pressure. A beam of light from a fixed source after reflection from the mirror will therefore turn in a vertical plane through an angle proportionate to the pressure, and in consequence of the rocking motion given to the frame D and the mirror about a vertical axis, the said beam will also turn in a hori- Zontal plane through an angle proportionate to the motion of the engine piston. If the beam of light be focused, after reflection, upon a ground glass screen, the resulting spot of light, will, in consequence of the two combined motions of the beam, travel in a visible path which is the indicator diagram of the engine, the abscissze being proportional to the travel of the engine piston and the ordinates to the corresponding pressures. In order to facilitate the observation of the diagrams thus obtained I resort to the methods such as illustrated in Figs. 9 and 10, which are hereinafter described.

The means for holding the spring E in the frame D are such that it is held at definite points, so that the spring is able to slide, in the direction of its length and cannot therefore be put into excessive tension by unequal expansion of the frame D and the spring E. In the method of holding the spring shown in Figs. 1 and 3 these objects are obtained by passing the spring E through slots in the plates F, F. The spring fits easily in these slots, and screws F, F, press on its upper surface so as to cause it to rest with slight pressure on the bottom of the slots. These screws also bend the spring slightly. The spring is thus held at four definite points, two above and two below, those above not being opposite to those below, and the pressure on these points being suficient to hold the spring in position, but leaving it free to expand and contract longitudinally.

The method of producing the pressure motion may be considerably varied without departing from the essential principles of my invention. For example, the spring E may be held only at one end, instead of at both ends, or a spring of other form, such as a spiral spring, may be substituted for it. Again, the means of communicating tlie motion of the spring to the mirror may be me dified In the form of apparatus illustrated in Figs. 1 to 3 the piston is only able to exert pressure upon the spring greater than atmospheric pressure and consequently the apparatus will not register pressures below the atmospheric. For many purposes tlis is suflicient, but in some cases it is desired to indicate suction pressures as well as compression pressures and to meet such cases I arrange that the piston shall be able to exert a pull as well as a push upon the spring in the manner shown in Figs. 6 and 7. Tie piston C is firmly attached by means of the screw T to the block or stirru U. Tl'zrough a square hole in the latter the spring E is passed. he screw V is adjusted so as to leave a very small clearance between its end and the spring E. The spring E is then free to move slightly within the block U, so that the piston is enabled to take its proper axial position without any lateral pressure wh'eh would cause it to bind in the bore in which it slides. In this case the attachment of the spring to the mirror is placed slightly to one side of the piston as shown in Fig. 6. A ball W may be placed underneath the spring in the block U in order to facilitate the relative movement of the spring and the piston.

In cases where the vibration of the engine hen running is excessive and causes angular motion of the mirror comparable with that caused by the pressure, I make use of two mirrors in the manner shown in Fig. 8. One of these is the ordinary mirror I of the indicator; the other I is fixed to the frame, its plane being approximately at right angles to that of tlxe mirror I. Light from a fixed source falls upon the mirror I as shown by the dotted line with arrows; it is then reflected onto the mirror-I and thence again reflected. The beam as finally reflected will receive the piston movement and the pressure movement just as in the case of the type of indicator illustrated in Figs. 1 to 3, but inasmuch as rotation of the apparatus as a whole about an axis parallel to that of he mirror I, will be accompanied by an equal rotation of the mirror I, such rotation will produce no change in the direction of the beam as finally reflected from I; just as in a sextant the reflection from the two mirrors in succession is unaltered by movement of the instrument as a whole. 1

Thus in tee indicator the movement observed in the reflected beam will be due to the pressure only, any movement of the mirror caused by vibration or by shake of the apparatus on the block A being eliminated by the use of the two mirrors.

It will be understood that, although for convenience of description the indicator has been described as with a vertical axis so that he movements of the mirror are respectively horizontal and vertical, this vertica. position of the axis is not essential. The indicator may be placed in any convenient position and the piston movement of the mirror will be about the axis of the bore, and tie pressure movement about an axis at right angles thereto.

Referring now to Figs. 9 to 13 particularly to 9, the beam of light coming from a fixed luninous point or source a, falls upon 1. i J p v r V I L r i the aim osaid plane mirror I. Ihe renected beam is caused to fall upon a com ex lens Z, placed at a distance from the mirror equal to its own focal length, and to strilre said lens at the point 0. T his lens must be of such size that in the extreme positions of the min determined by the varying motions of the engine piston and the pressure within the c i der, the beam always stril'es the lens. lens causes the parallel beam ol' light to i i verge to a point (X which, since the sour distant, will be a little beyond the focal plane of the lens. Since the mirror is a 1 the other principal locus of the lens the direction of the beam 0, (Z alter passing the lens, is parallel to the axial line I f joining the center of the lens to the mirror, and it a semhopaque screen were placed in the plane d j'tlie indicater diagram of the engine would be traced thereon. In order to make this dia' "an-r readily visible, I do not employ a ,and glass or other semi-opaque screen in the plane tlf, but l place a second lens Z at a distance from the plane (Z f equal to its own focal length, which is short enough to give considerable magnification. This second lens is of approximately the same diameter the first so that the rays from the first lens meet it in all positions of the mirror. Altar passing the second lens the beam of lie' i c d 0 becomes a parallel be and this parallr beam meets the axis I f at the point 7 which i. a principal focus of the second lens. At this point the eye of the observer is placed and sees through the eye piece P the magnified image of the diagram formed in the plane (if. Since practically no light is lost in the pas-- from the mirror to the eye, a very w source of light is suflicient to make the i lagram visible.

To provide [or the measurement oi" the diagram, I mayplace in the plane (Zf a transp ent screen Z Figs. 11, 12 and havin ured upon its surface horizontal and TO '0 scales. These scales will be seen at the same time as the diagram and magnified to the same extent, and the diagram can accordingly be very easily compared with them.

The source of light, lenses and screen may conveniently be carried adjustably on the same frame X which may be mounted on a stand or pillar X which is adjusted in position with respect to the engine cylinderand the mirror indicator fitted thereto, until the correct position for obtaining and viewing the diagram is found.

Although in the above description and in Fig. 9, I have shown a plane mirror with a point source of light and have shown the first lens placed at a distance from the mirror equal to its own focal length, and the second lens at a distance from the focal plane of the first equal to its focal length, these details are not essential to my invention, which, in its broader scope consists in catching the rays reflected from the indicator mirror up on a lens placed at some considerable distance from the mirror, and in collecting the rays emerging from the lens by means of a second lens which directs them to the eye of the observer.

Referring now to Fig. 10 in this case the rays from the source of light a fall upon a convex lens Z before reaching the mirror I. A second lens Z is placed at such a distance from the mirror and is of such focal length that the point g, which is the focus of "his lens conjugate to the mirror I, will be distant about fifteen inches from the lens. This lens Z is, as in the previous case, of such diameter that the beam of light reflected from the mirror always falls upon it. The lens Z is of such focal length and is so placed in relation to the mirror B and the luminous point or source A, that the rays from that point or source come to a focus near to the lens Z after reflection from the mirror, that is to say, the distance al and the sum of the distances Z l and I Z are conjugate focal distances for the lens Z. Under the arrangement described, if a ground glass screen were placed in the position of the lens Z it would receive the indicator diagram of the engine in the well known manner; the effect however of substituting the lens Z for the screen is to refract the beam falling upon the latter, in any position ofthe mirror, to the point g on the axis of the lens, at which point the eye of the ob server is placed. Since the distance of this point from the lens, (viz :about fifteen inches) is a convenient distance for focusing the eye, and since the rays composing the beam in any position proceed from a point on the lens as shown in the figure, the d gram will be visible as a bright line of light. A screen bearing scales as in the previous case may be placed close to the lens Z with which scales the diagram may be compared as before stated. In place of the convex lens Z and a plane mirror I a concave mirror might be used of such focal length as to bring the beam from the source a to focus upon the lens P.

It is obvious that these methods of observing diagrams may be applied to any form of indicator in which a beam of light from a mirror is caused to move in one direction in accordance with the pressure and in a perpendicular direction in accordance with the motion of the engine piston. The said methods may also be used in connection with mirror indicators employed with other apparatus than engines, that is to say, any apparatus that will operate to impart continuous cyclical motion to the mirror about two axes at right angles.

in using the apparatus in connection with internal combustion engines an electric spark gap a (Fig. 11) may be arranged in series with the igniting spark in the engine and close to the source of light. An image of the spark at the time when it passes is then formed near that of the source of light and this image is easily seen by the observer; the timing of the spark can thus be investigated very conveniently.

What I claim and desire to secure by Letters Patent of the United States is 1. in apparatus for optically producing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a spring-controlled piston located in said bore, a frame mounted on said conical block, a mirror pivotally mounted on said frame, means also carried by said frame whereby said piston imparts angular displacement to the mirror in one plane, and means whereby said mirror is rocked about the axis of said piston in a plane perpendicular to the first mentioned plane, for the purposes specified.

2. in apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame located axially with respect to said piston, a mirror pivotally mounted on said frame, a bar spring mounted transversely in said frame so as to bear loosely on said piston, and to be deflected thereby at its center, means for oscillating said frame about the axis of said piston in one plane, and means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane for the purpose specified.

3. In apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame located axially with respect to said piston, a mirror pivotally mounted on said frame, a bar spring ada ted to bear loosely on said piston and to co deflected thereby, means whereby said spring is mounted transversely in the frame at four points so as to be free at the ends and capable of slight axial movement, means for oscillating said frame about the axis of said piston in one plane, and means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane for the purpose specified.

4. in apparatus for optically producing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a pis on located in said bore, a frame mounted on said block, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to bear loosely on said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, and means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, for the purposes specified.

5. In apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, a bar spring mounted transversely on said frame so as to bear loosely on said piston and to be deflected thereby at its center; means also carried by said frame whereby said spr ng imparts angular displacement to the mirror in one plane, means for oscillating said frame about the axis of said piston in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, and means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, for the purpose specified.

6. in apparatus for optically producing indicator diagrams, the cmnbination of a spring-controlled piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, means also mounted on said frame whereby said piston imparts angular displacement to the mirror in one plane, means for oscillating said frame about the axis of said piston in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, and means for measuring said diagram by observation, for the purpose specified.

7. in apparatus for optically producing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a piston located in said bore, a frame mounted on said block, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to bear loosely on said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, and means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, for the purpose specified.

8. In apparatus for optically reducing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a piston located in said bore, a frame mounted on said block, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, and means for measuring said diagram by observation, for the purpose specified.

9. In apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about tie axis of said piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpeiiidicular to the first mentioned plane, a source of light directed onto said mirror, and a lens situated in a position to receive the beam of light reflected from the mirror during all positions of the latter and to direct the same to the observers eye, for the purpose specified.

10. In apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, a lens situated in a position to receive the beam of light reflected from the mirror during all positions of the latter and to direct the same to the observers eye, and means for measuring said diagram by observation, for the purpose specified.

11. In apparatus for optically producing indicator dia rams, the combination of a spring-control ed piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, means also mounted on said frame whereby said piston imparts angular displacement to the mirror in one plane, means for oscillating said frame about the axis of said piston in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, and a transparent screen engraved with scales and placed in the plane on which the image of the source of light is formed, for the purpose s ecified.

12. n apparatus for optically producing indicator diagrams, the combination of a piston subject to varying pressure, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, a lens situated in a osition to receive the beam of light reflectec from the mirror during all positions of the latter and to direct the same to the observers eye, and a transparent screen engraved with scales and placed in the plane on which the image of the source of light is formed, for the purpose specified.

13. In apparatus for optically producing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a piston located in said bore, a frame mounted on said block, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of the piston in one plane, means whereby said spring imparts angular displacement to the mirror in a lane perpendicular to the first mentioned p ane, a source of light directed onto said mirror, a convex lens situated in a position to receive the beam of light reflected from the mirror during all positions ofthe latter and to direct the same to the observers eye, and a transparent screen engraved with scales and placed in the lane on which the image of the source of light is formed, for the purpose specified.

1a. In apparatus for optically producing indicator diagrams, the combination of a spring-controlled piston subject to the varying pressure in the interior of an internal combustion engine, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, means also mounted on said frame whereby said piston imparts angular displacement to the mirror in one plane, means whereby said mirror is rocked about the axis of said piston in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the ob server's eye, and an electric spark gap arranged in series relationship with the igniting spark and placed in close proximity to the source of light, for the purpose specified,

15. In apparatus for optically producing indicator diagrams, the combination of a piston subject to the varying pressure in the interior of an internal combustion engine, a frame mounted axially with respect to said piston, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of said piston in one plane, means whereby said spring imparts angular dis lacement to the mirror in a plane perpendicu ar to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, means for measuring said diagram by observation, and an electric spark gap arranged in series relationship with the igniting spark and placed in close proximity to the source of light, for the purpose specified.

16. In a paratus for optically producing indicator diagrams, the combination of a conical block having a bore communicating with the source of pressure, a piston located in said bore, a frame mounted on said block, a mirror pivotally mounted on said frame, a spring mounted transversely in said frame so as to engage with said piston and be deflected thereby, means for oscillating said frame about the axis of the piston in one plane, means whereby said spring imparts angular displacement to the mirror in a plane perpendicular to the first mentioned plane, a source of light directed onto said mirror, means whereby the reflected rays that trace out the diagram are directed during all positions of the mirror to the observers eye, means for measuring said diagram by observation, and an electric spark gap arranged in series relationship with the igniting spark and placed in close proximity to the source of light, for the purpose specified.

In testimony whereof I aflix my signature in presence of two witnesses.

BERTRAM HOPKINSON.

Witnesses T. A. G. COOHERTON, E. FREEMAN.

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