THE CALORIMETER SYSTEM AND MEASUREMENT OP HEAT. 165 



means of a cord over a pulley, though the still cruder means of filing a 

 given weight of iron filings from a piece of cast iron was used in one 

 of the preliminary experiments. In both of these methods obviously 

 but very crude estimates as to the actual amount of external muscular 

 work performed could be made. As measures of relative rather than 

 absolute amounts, they were less objectionable, but at best they were 

 far from the accuracy that has been striven for in the development of 

 the respiration calorimeter and its accessory apparatus. 



It was observed that the greatest amount of work, with the minimum 

 fatigue, could be performed on a bicycle, and accordingly an ergometer 

 was constructed in which a pulley attached to the armature shaft of a 

 small dynamo was braced against the rear tire of a bicycle wheel. 1 This 

 instrument could be calibrated but roughly, to be sure, but did, how- 

 ever, serve its purpose in the transitional period during which the 

 bicycle ergometer described beyond was in process of development. 



Retaining the bicycle form so that the bulk of the work is done by 

 the powerful leg muscles, the present ergometer consists of an arrange- 

 ment for rotating a heavy copper disk, corresponding to the rear wheel 

 of a bicycle, in the field of an electro-magnet, which thus gives the effect 

 of an electric brake. The apparatus is shown connected ready for use 

 in figure 47. 



The principle is the well-known one of magnetic induction. A current 

 of electricity is passed through the field coils of the magnet, and when 

 power is applied to the pedals of the wheel and transmitted to the revolv- 

 ing disk, it is transformed into heat. To calibrate the apparatus, it is 

 put inside the respiration chamber in such a way that the axle of the 

 wheel is connected to a shaft which passes through the food aperture 

 and is revolved by power applied outside. The rate of revolution is 

 shown by a cyclometer. The strength of the magnet is determined by 

 the electric current through the coils, which is measured. With a given 

 strength of magnetization the power applied to the pedals and conse- 

 quent heat generated will vary directly as the speed of revolution ; the 

 heat is therefore measured for different rates of speed. The data thus 

 obtained show the amounts of energy transformed per revolution with 

 the given magnetization. The mechanical friction in the ergometer per 

 revolution is constant and included in the calibration. Accordingly, 

 when the man is working on the ergometer, the number of revolutions 

 as recorded by a cyclometer multiplied by the energy per revolution 

 gives the muscular work done at the pedals. We believe the measure- 

 ments are accurate within a fraction of i per cent. The apparatus 

 has proved very satisfactory. 



'U. S. Dept. of Agr., Office of Experiment Stations Bull. 136, p. 31. 



