132 A RESPIRATION CALORIMETER. 



The end of the soft iron core of the magnet is surrounded by a brass 

 cap, which gives a rounded surface at the end of the coil, so that when 

 the armature settles into position it will easily slide along the end of 

 the magnet and not catch at any point. The current used to magnet- 

 ize these fields is taken from the observer's table (see p. 136), and has 

 in series with it two i6-candlepower no-volt lamps, one of them being 

 the galvanometer lamp. The strength of current through the two 

 magnets is varied by means of the resistance coil R shown in figure 35. 



The equipoise beam in its descent touches the wire W, used to deflect 

 the water current, thereby closing an electric circuit and causing an 

 electric signal-bell to ring continuously until the operator lifts a switch 

 on the observer's table. In operating the meter, then, the only care 

 required on the part of the observer is to see that the readings of the 

 pointer on the dial are accurately recorded each time the bell rings. 



Calibration of the meter, The meter was calibrated by weighing the 

 total amount of water delivered from each can, noting carefully the 

 position of the pointers. For this purpose a large enameled-ware pot 

 with a hard-rubber cover was accurately weighed on the large balance 

 (p. 56). A specially constructed funnel was placed under the overflow 

 and siphon of one of the cans and the neck of the funnel inserted in a 

 hole in the cover of the previously weighed pot. When the can was 

 released the water was delivered into the weighed pot instead of into 

 the drain. After all the water had drained out of the can and funnel, 

 the pot, plus the water, was weighed. These weights were usually 

 made to the tenth of a gram. A curve was plotted which showed the 

 weights of water delivered by the meter with the pointer at different 

 positions, and consequently it is now only necessary for the observer to 

 record the position of the pointer. 



Accuracy of the meter. The extreme accuracy of this water-meter 

 has been surprising, for while the amounts as indicated by the readings 

 on the dial may be from i to 2 grams either side of the true amount 

 delivered, these differences tend to counterbalance each other, and it is 

 safe to state that in a series of observations 10 cans full, or 100 kg. of 

 water, will be weighed to within a very few grams. 



To facilitate in ascertaining the weights of water remaining in the 

 can at the end of a short period in which less than 10 kg. of water 

 were flowing, a water-gage graduated in liters is attached to the front 

 of each can. It is thus possible for the observer at the end of a period 

 to note the quantity of water in the can to within a tenth of a liter. 

 A more exact estimate of this amount can be made by noting the time 

 between the emptying of the last can and the end of the period. 



