394 THE OUTGO OF ENERGY 



against different parts of the skin of the hand, 

 arm, and face. Select hairs which when pressed 

 against the skin of the respective regions give no 

 sensation %f pressure. Shorten the hairs until 

 the pressure is just perceptible. This will be the 

 "pressure threshold." Make a loop in a short 

 silk thread and pass the loop about the lever 

 exactly one millimetre from the axis. Hang on 

 the end of the thread a light bent hook. Coun- 

 terpoise the lever very exactly, so that the 

 slightest force applied to the end of the straw 

 will raise the lever from the after-loading screw. 

 By counterpoising in this way, the lever becomes 

 a balance. On the bent hook hang a ring of 

 German silver wire weighing one decigram (0.1 

 gram). Find a point on the lever 100 mm. from 

 the axis. The weight of one decigram suspended 

 1 mm. from the axis of the lever will be raised 

 by a force of T ^ of a decigram, equal to one 

 milligram (0.001 gram) applied 100 mm. from 

 the axis. At 50 mm. from the axis, 0.1 gram, 

 suspended 1 mm. from the axis, will be lifted by 

 a force of -^ gram (0.002 gram). Find the dis- 

 tance from the axis at which each testing-hair, 

 when pressed vertically against the lever, will 

 just fail to lift the lever; in other words, the 

 point at which the pressure will be just sufficient 

 to bend the hair. The number of millimetres 



