FROM THE PRACTICAL VIEWPOINT 



recorder is in reality a pressure recorder only in 

 which the changes in pressure due to varying tem- 

 peratures are so graduated as to indicate with 

 fair accuracy the temperature of the milk. With 

 the ether instrument, the amount of movement of 

 the arm for, say, a ten degree rise, changes as the 

 higher temperatures are reached. So the grad- 

 uations on the chart are not of the same width all 

 the way across. In the mercury instruments this 

 is not true, but the degree graduation lines are 

 equally distant one from the other. There are cer- 

 tain inaccuracies in these instruments which must 

 be taken into account. Thus it can be seen that 

 the temperature of the room in which the recorder 

 is placed must have an effect upon the pressure 

 coil. In the instrument shown in Fig. 30 an at- 

 tempt is made to correct this by attaching to the 

 arm a compensating coil. This coil also contains 

 mercury, and is so placed that it moves in a di- 

 rection opposite to that of the coil to which the 

 capillary tube is connected. The recording arm 

 is attached to both coils. When, therefore, both 

 coils move as the result of the room temperature, 

 no effect is produced upon the recording arm, but 

 it remains stationary. Any increased pressure in 

 145 



