Page:Popular Science Monthly Volume 58.djvu/219

Rh might perhaps lend itself to such an inquiry.

In the experiments referred to, the man under investigation received daily a known quantity of potential energy in the form of food. Part of this was converted into external mechanical energy and was measured; of the remainder, part appeared as heat and part was carried away in the refuse products of the body. The internal work of the body is ultimately converted into heat, and appears in the total heat of radiation and respiration. Thus energy is expended in causing the heart to beat and the blood to circulate and the lungs to expand. This internal work is not stored up, but is transformed into heat and radiated away with that which results directly from combustion. But external work done, like turning a grindstone or sawing wood, is not represented in the heat radiations of the body.

In order to do the desired amount of work within the calorimeter, the man operated a stationary bicycle, which was geared to a small dynamo. The front wheel of the bicycle was removed, and the rear wheel served as a driving pulley for the dynamo. The latter generated a current, the energy of which was measured by an ammeter and a voltmeter. When this current passed out of the calorimeter, its energy was not included in the heat measured by the calorimeter. But in some cases the current flowed through an incandescent lamp inside the calorimeter. Then the mechanical energy done by the man was all turned to heat within the calorimeter; part of it through friction in the bicycle and dynamo, part through the electric current which flowed through the lamp. The former was measured as accurately as possible by seeing how much energy was required to drive the bicycle when using the dynamo as a motor, supplying current to the latter from a battery and measuring the energy so supplied by an ammeter and volt-meter. The quantity of heat resulting from this friction must be subtracted from the total heat measured, in order to ascertain the quantity which was given off from the man's body directly as heat. And in those cases where the electric lamp was inside the chamber (and hence the work done by the subject was converted into heat within the chamber) this total amount must be subtracted from the heat measured to give the amount of heat given off as such by the subject of the experiment.

Thus we measure the quantity of external work done; but nothing is here learned about the internal work. The latter is converted into heat within the body and, when radiated away, is measured with the rest by the calorimeter. The amount of external work done in driving this bicycle-dynamo combination in one of the experiments (which continued for 96 hours) was equivalent to 256 large calories per day. This was about 40 watts for eight hours, or 788,000 foot-pounds, or 394 foot-tons. The total quantity of energy yielded was 3,726 large calories on the average for each of the four days. Since 256 is about 7 per cent, of 3,726, we see that the man converted 7 per cent, of the energy contained in his food into mechanical energy, 93 per cent, appearing in the heat of radiation and respiration. This gives the man, regarded as a machine for doing mechanical work, a 24hour efficiency of 7 per cent. During the eight hours in which work was done the total consumption of energy was about 1,850 calories. Dividing the work done by this figure, we have for the mechanical efficiency during working time, 14 per cent. But there is still another way of reckoning this efficiency. Inasmuch as a large part of the energy supplied to the body would have been required to do internal work and keep the body warm, if no work had been done, we can fairly charge against the work done only the excess of energy supplied during the days when work was done over that required by the same man when no appreciable external work was done. The average quantity of energy