THE MEASUREMENT OF POWER 21 



words, a plowing tractor should have the highest possible 

 tractive efficiency consistent with other vital features. Tract- 

 ive efficiency is properly estimated on the basis of the ratio be- 

 tween the drawbar, or tractive horsepower and the brake h.p. 



Tractive horsepower is ascertained by noting the speed of 

 travel and determining the resistance of the load by means of 

 a traction dynamometer. This instrument consists fundamen- 

 tally of a calibrated spring which may be attached between 

 the engine and its load. The tension on the spring causes it 

 to move a pointer upon a dial that is graduated to show the 

 resistance in pounds. There may also be a mechanism operated 

 by clockwork for driving a graduated tape at a speed propor- 

 tioned to the duration of the test. A recording pencil attached 

 to the pointer traces an irregular autograph, the average dis- 

 tance of which above the base line gives the average draft 

 during the tests. The distance traveled in a given time, 

 together with the resistance, gives the tractive h.p. 

 For example : A tractor moving at the rate of two miles per hour, 

 travels in one hour 10,560 feet, or 176 feet per minute. If the 

 average resistance or draft is 7,500 pounds, then in one minute 

 the tractor does 1,320,000 foot-pounds of work. This divided 

 by 33,000 gives 40 h. p. as a result. By cancellation of constant 

 factors we arrive at a much shorter formula, which is: Speed 

 in miles per hour X drawbar pull -*- 375 = tractive h.p. 



The object of a heat engine is to convert the chemical energy 

 of the fuel into mechanical energy. All other things being 

 equal, the most efficient motor is the one which will deliver the 

 largest amount of this energy as useful work. In order to 

 compare engines in this respect it is necessary to reduce the 

 energy supplied in the fuel and in the work recovered to the 

 same basis and thus determine the thermal efficiency of the motor. 

 In order to determine the heat value of different fuels, engineers 

 have adopted units of comparison. For English-speaking 

 countries the standard is the British thermal unit (B.t.u.), 

 which is the amount of heat necessary to raise the temperature 



