Improved Respiration Calorimeter 343 



successful for measuring work done with the muscles of the legs. The 

 principle of the device is that of the electric brake. It is designated a 

 "bicycle ergometer," since it bears some resemblance to a bicycle; in 

 fact, in its construction all of a bicycle except the wheels was used, and 

 the work done in operating it is of the same kind as that involved in 

 propelling a bicycle. In the ergometer, however, the front wheel of the 

 bicycle is replaced by a pedestal and the rear part of the frame is sup- 

 ported by a rack, so that a heavy copper disk, 40.5 cm. in diameter and 

 approximately 6 mm. in thickness, which replaces the rear wheel of the 

 bicycle, will rotate freely on its ball-bearing axis. An electromagnet is 

 attached to the frame, with its poles on opposite sides of the disk, with 

 the inner edge of the rectangular-pole faces coincident with the circum- 

 ference of the disk, and with the face of each pole i mm. from the surface 

 of the side of the disk. When there is no current in the field coils of the 

 magnet, the amount of energy required to cause the disk to rotate between 

 the pole faces is small, being merely that necessary to overcome the 

 friction of the bearings and the resistance of the air against the moving 

 parts; but when there is a current in the coils, with its resulting magneti- 

 zation, currents are induced in the disk rotating in the magnetic field, 

 which tend to prevent it from rotating. The brake effect depends upon 

 the flux density of the field, which varies with the strength of the mag- 

 netizing current. The amount of work done by the subject on the 

 ergometer is therefore easily controlled by adjusting a rheostat in 

 series with the coils of the magnet until the strength of the current is 

 that which will result in the desired resistance to be overcome in causing 

 the disk to rotate. A particular advantage in this apparatus is the 

 constancy and uniformity with which the effect may be reproduced. 



The power applied to the pedals when work is done on this ergometer is 

 converted into heat, a small part of it by the friction of the moving posts 

 of the mechanism and the rest by the energy transformations in the disk. 

 The quantity of heat thus produced varies with the intensity of the 

 magnetic field and also with the rate of rotation of the disk. From 

 calibration of the ergometer in the calorimeter the amount of heat pro- 

 duced by each rotation of the disk in the magnetic field was determined 

 for a considerable variety of conditions of velocity of the disk and strength 

 of magnetizing current within the range commonly employed in experi- 

 ments. By use of curves derived from the data of calibration the heat 

 equivalent of the external muscular work performed by the subject on 

 the bicycle ergometer is computed directly from the total number of 

 rotations of the disk, as shown by an automatic counter, and of strength 

 of current in the field coils, as shown by an accurate ammeter. 



The heat produced in the ergometer is measured by means of the 

 calorimeter, together with that eliminated from the body; but since the 

 former can be ascertained, as just explained, it may be subtracted from 

 the total heat measurement, when the amount of heat produced by the 

 subject in performing muscular work is computed. 



