INTRODUCTION 7 



In constructing the magnet we were at a disadvantage in not knowing 

 the exact dimensions of the magnet in the first ergometer, and since no 

 records were available as to the size of the wire, we were obliged to make 

 an approximate estimate. It was found subsequently that in order to 

 secure a sufficiently strong magnetic field with a moderate current, it was 

 necessary to rewind the magnet with a smaller size of wire (No. 19 B. & S. 

 gauge), 0.91 mm. diameter, thus increasing the total resistance to 10 ohms. 



With this winding of the magnet it was found that a current of 1.5 

 amperes through the coil produced substantially the same drag effect as 

 did 1.25 amperes on the older machine. The new machine has a dis- 

 advantage in that it develops a larger amount of heat in the magnet coil 

 itself than the first ergometer, thus making a somewhat larger correction 

 to be deducted from the total heat measured if the apparatus is used 

 inside a calorimeter. As a matter of fact, with a current of 1.5 amperes 

 through the field, the heat development in the magnet itself is 17.8 to 17.9 

 calories per hour, while with the older form of apparatus with a current 

 of 1.25 amperes through the field, the heat development was 10.9 calories 

 per hour. 



GENERAL CONSIDERATIONS REGARDING METHOD OF USE. 



When the ergometer is in use, a variable resistance, which consists 

 of German-silver or manganin wire, is placed in series with the magnet coil 

 and the mil-ammeter. By altering the variable resistance, the current 

 is kept constant throughout any experiment. At the beginning of an 

 experiment the coil is cold, and if the adjustable resistance is set at a 

 given point the current passing through the magnet gradually decreases 

 as the field begins to warm up; it accordingly becomes necessary to ad- 

 just the resistance until the desired amount of current through the field 

 is obtained. In approximately 20 minutes to half an hour constant 

 temperature conditions are obtained and thereafter no material adjust- 

 ment of the resistance is necessary. The larger sprocket has 26 teeth 

 and the smaller sprocket 8, the ratio being 1 to 3.25. While one can com- 

 pute from the number of revolutions the distance that theoretically 

 would have been traversed had the subject been riding a bicycle with a 

 standard wheel of 28 inches, this really has very little significance, as of 

 course in riding a stationary bicycle there is no wind resistance and there 

 is no energy expended on tires; consequently it is necessary to supply 

 sufficient resistance to the copper disk to compensate for the absence of 

 these factors. 



With this ergometer it was possible within certain limits to vary the 

 amount of work done per hour by varying the speed of revolution of the 

 disk. But few riders care to ride for any length of time at less than 50 

 revolutions of the pedal per minute, and tests on a number of individuals 

 have shown that the revolutions usually ranged from 55 to 80 per minute; 

 with highly trained professional bicyclists, the rate of revolution may rise 



