. Electrical news and engineering . ld be marked at one-half the initial is best to use as light a spring as the pressure will allow, thusmaking the card as large as possible, for the larger the card thegreater the defects will be shown. You must also exercise greatcare to get the drum motion to exactly represent the pistonmovement. This is difficult to do in high-speed engines. Thebest test is the length of the card ; if it is longer when taken atfull speed than when taken at the slowest speed the engine canbe run at, then there is something wrong, and you will not get acorrect ca
. Electrical news and engineering . ld be marked at one-half the initial is best to use as light a spring as the pressure will allow, thusmaking the card as large as possible, for the larger the card thegreater the defects will be shown. You must also exercise greatcare to get the drum motion to exactly represent the pistonmovement. This is difficult to do in high-speed engines. Thebest test is the length of the card ; if it is longer when taken atfull speed than when taken at the slowest speed the engine canbe run at, then there is something wrong, and you will not get acorrect card. After having blown out the pipes with steam, put on the indica-tor and attach your cord, adjusting its length so that your cardis exactly in the centre of your paper. Now unhook your cordand put on your paper, being sure that it is even at the cornersand tight around the drum. You are now ready to take yourcard. Start up your drum by hooking on your card. Now turnon the steam by opening the cock at the bottom of the indicator ;. leave this cock open a few seconds that the instrument may bewarmed up, then shut off the steam and draw the atmosphericline ; then open the cock and apply the pencil to the drum bypushing il towards the paper and holding it firmly against thestop on the instrument just sufficient time for the engine to makeone revolution. You will then have a clean, fine-lined card—Iheonly correct card to measure up the power of the engine from. Having taken the diagram with our indicator correctly adjusted,and the connection properly made, we now want to know what isthe h. p. as shown by the diagram. To find this it is merelynecessary to measure the space enclosed by the diagram, and bythis means get the mean effective pressure exerted upon the pistonthroughout its stroke, which this area really represents. Thispressure being multiplied by the number of square inches of thepiston area gives us the total pounds of force exerted, and as thisforce is acting through
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