286 Mr. H. F. W. Burstall on the Measvrement 



As the working stroke in the Otto cycle only occurs once in 

 four strokes of the piston, and as the temperature had to be 

 measured at particular parts of that stroke, it was necessary 

 that the galvanometer should only he in circuit at the. right 

 moment, otherwise the galvanometer swing would not give 

 an indication of the true temperature of the wire. During 

 each explosion stroke the measuring-wire goes through the 

 whole cycle of temperature and, in order to secure a parti- 

 cular epoch of this stroke, the following arrangement of con- 

 tact makers (shown in fig. 3) was designed. The galvanometer 

 circuit was broken in two places, one of which was made 

 during every working stroke, and the other at a definite 

 epoch at every revolution ; thus the galvanometer circuit was 

 only completely made at one definite point in every working 

 stroke. At any required point in the stroke of the piston, 

 contact is made every revolution by a cam (C) on the crank- 

 shaft of the engine lifting one end of a vibrating lever, de- 

 pressing a wire (B), fixed to the other end, into a mercury 

 cup D, and thus making a galvanometer contact for a 

 definite time (about -£ G of the stroke) at a certain fraction of 

 the stroke. This, however, makes contact once every revo- 

 lution, while the temperature is measured only during an 

 explosion stroke. To ensure that the galvanometer circuit shall 

 only be made during every working stroke, it is again broken 

 and only made at the proper time by a relay. This relay is 

 on a separate circuit which is completed by means of a steam- 

 engine indicator (P), whose pencil is replaced by a metal 

 point. During the compression and exhaust strokes the pencil 

 bears on a boxwood cylinder, but when the explosion occurs 

 the arm is forced upwards by the pressure and the pointer 

 makes contact on a strip of brass Q, thus completing the relay 

 circuit. The electromagnet is excited and the beam de- 

 pressed, completing the galvanometer circuit through the 

 mercury cups E and S. As soon as the relay has moved 

 through a small distance, the indicator break is short-circuited 

 by the mercury cup T, and thus the relay circuit remains 

 closed until broken by a contact on the exhaust-valve of the 

 engine. Although somewhat complicated this relay was 

 found to work well, but great care had to be taken that the 

 contacts were all well amalgamated and the mercury clean, 

 otherwise the very small currents in the galvanometer circuit, 

 when a balance is nearly obtained, are entirely broken by any 

 dirtor grease. 



To make a determination of the temperature at any 

 required point the contact-maker on the engine-shaft was 

 set, by means of marks on the driving pulley, to make con- 

 tact at the required position of the piston. An approximate 



