THE CALORIMETER SYSTEM AND MEASUREMENT OP HEAT. 131 



chamber before the water actually runs out of the system, ample time 

 is given to record the exact position of the pointer on the dial. After 

 this position is recorded, the apparatus requires no more attention until 

 the next can is filled. As soon as about 400 grams of water have run 

 out of the system, the equipoise settles back to the position shown in 

 the diagram, the bulb projection on the bottom of the counterpoise 

 resuming its position between the jaws of the spring. 



As a result of the impetus given the system by the spring clutch, 

 and that in the opposite direction by the balance-spring, the momentum 

 of the large mass of metal and water has a tendency to cause the sys- 

 tem to oscillate for several seconds before finally assuming a position of 

 equilibrium. Preliminary experiments showed that this motion per- 

 sisted a considerable time longer, indeed, than the 20 seconds required 

 for the water to flow into the lower can and begin to run out of the 

 system. Consequently some method was necessary to check this oscil- 

 lation and have the system attain equilibrium as rapidly as possible. 

 To accomplish this end, an iron armature ( in fig. 35) was fastened to 

 one of the connections between the upper and lower cans. An electro- 

 magnet was fastened to the upright wooden frame supporting the whole 

 system in such a position that when the can was released and was 

 vibrating back and forth the iron armature would rub over the end of 

 the electro-magnet. By having a feeble current passing around the 

 magnet, the movement of the can could be very readily checked. 



It was found, however, practically impossible to regulate the strength 

 of the current so as to retard the vibration and yet not hold the armature 

 against the end of the magnet, and thereby prevent the system from 

 swinging freely and being weighed accurately. A circuit-breaker was 

 devised and attached to the shaft supplying power to the calorimeter 

 room. (See fig. i . ) By this simple device the current is made and broken 

 every few seconds indeed, at approximately such times as would rep- 

 resent the end of the vibration. The effect, therefore, is to have the 

 armature attracted by the magnet and held firmly for an instant ; the 

 current is then broken and the system begins another oscillation, at the 

 end of which the current again holds the system for an instant, the 

 effect being to diminish the momentum each time the armature is in 

 contact with the magnet. Finally, by the observer's raising a switch 

 and thus completely breaking the current around the magnet, the can 

 swings freely and may be weighed accurately. By means of the two 

 nuts on the central rod of the magnet the distance of the ends of the 

 magnet from the armature when the system is in equilibrium can be 

 altered at will. 



