22 



A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



error. The value reported by Benedict and Carpenter, 1 as found in the 

 one experiment made, was 0.001547 calorie per revolution. This repre- 

 sents about 8 per cent of the total heat per revolution with a current of 

 1.25 amperes. Unwarranted use of this figure was made by the authors 

 in their discussion of the problems involved in experiments on men, but 

 the best data then available were used. That this value was always too 

 high seems obvious, and at the earliest opportunity a measurement of 

 the heat of friction given off by this machine was made in the chair 

 calorimeter at the Nutrition Laboratory. The results of two experiments 

 in June 1911 are given in table 4. 



Table 4. Results of friction tests of ergometer I. 



[Magnet not excited.] 



These values, while by no means as concordant as could be desired, 

 are but approximately one-fifth to one-tenth of that found in the single 

 test made in Middletown, Connecticut. Subsequent tests made with 

 ergometer II (see p. 29) indicate that these values are probably not far 

 from correct, though it should be stated that a calorimeter designed to 

 measure the heat production of a man is not best adapted to measuring 

 so small an amount as 1 or 2 calories per hour. 



Exactly what use of this value is justified in an experiment with a man 

 it is not the province of this paper to discuss. These tests are given 

 primarily to show that the earlier value was entirely wrong and hence all 

 calculations made with it should be regarded as worthless. 



CALIBRATION TESTS OF ERGOMETER II. 

 The second ergometer was constructed during the summer of 1911 

 and, after preliminary tests as to the winding of the magnets, the ap- 

 paratus was substantially installed in the chair calorimeter for a series 

 of tests. These tests covered wide ranges of speed and magnetizing 

 current. A further variant was introduced in that in some of the ex- 

 periments the relative position of the disk and pole-faces was changed 

 so that the disk rotated much nearer one pole-face than the other. Sub- 

 sequently, the entire magnet was moved towards the hub in a straight 

 line, so that in a few experiments the pole-faces were nearer the hub by 

 about 20 mm. In this new position the disk was at times in the center 

 of the space between the pole-faces and at other times it was as near as 

 possible to one of the pole-faces without actual contact with it. These 

 tests with varying positions of the magnet were all incidental to a study 



1 Benedict and Carpenter, loc. cit., p. 15. 



