Nov. 22. I9I5 Improved Respiration Calorimeter 321 



wound, and the direction in which the armature of the motor will rotate 

 depends upon the windings by which the fields are excited. With cur- 

 rent flowing in both pairs of field coils alike the armature will not turn 

 in either direction; but if one pair of coils is shunted, the effect of the 

 other pair predominates, and the armature will rotate. The direction 

 of rotation depends upon the closing of one or the other of two contacts 

 in the circuit of the field windings, thereby shunting one or the other 

 pair of field coils, and the duration of rotation depends upon the length 

 of time the contact is closed. 



The contacts are closed by keys which are depressed by cams on a 

 light cam shaft driven by a small motor. The cams rotate continuously, 

 and when a circuit is to be closed an idler swings between one of the 

 cams and the key to be depressed. There are three cams for each key, 

 differing in respect to the time each one presses on the idler, and the 

 duration of contact depends upon which of these cams is engaged. 

 Each idler is mounted on a lever which carries it into position between 

 the cam and the key, the lever being actuated by a pin on the rotating 

 shaft. A galvanometer needle decides which lever is to swing and to 

 which of the three cams it is to be carried. The galvanometer, though 

 incorporated in the device for making contacts, is connected with the 

 indicator on which is set the temperature at which the water is to be 

 kept. The direction and amplitude of deflection of the needle depend 

 upon whether and to what extent the temperature of the water is above 

 or below that set on the indicator. The galvanometer thus governs 

 the direction and extent of motion of the slider on the rheostat which 

 regulates the current in the heating coil in the water channel. The 

 period of the galvanometer is less than 3 seconds, and the cam shaft 

 rotates once every 4 seconds, so that changes in the temperature of the 

 water, when necessary, are made that often, giving practically contin- 

 uous regulation. With three different degrees of automatic adjustment 

 in either direction, and with the possibility of shifting the slider by hand, 

 the water flow may be quickly brought to any desired temperature and 

 easily maintained. This device for causing the movement of the con- 

 tact on the rheostat is shown in Plate XXXV, figure 2, which shows 

 also the rheostat and the motor for adjusting the rheostat. 



The indicator on which the desired temperature is set, which may be 

 seen in Plate XXXVI, figure 2, at the right of the water heaters, is a 

 special Wheatstone bridge, in one arm of which is the resistance ther- 

 mometer in the water channel. The resistance of the slide wire of this 

 bridge, which is nearly 45 cm. long, is sufficient to compensate for an 

 upsetting of the balance of the bridge due to the change in resistance of 

 the thermometer that would result from a change of 10 degrees in the 

 temperature of the water, and by means of a coil in series with the wire 

 the amount of balance resistance may be doubled. When the coil and 

 slide wire are in series, the range of the dial is from 12 to 24 degrees; 



