LESSON XXIV. HEARING. REACTION TIME. 



1. Reaction time for hearing. Arrange a drum to 

 rotate about 10 cm. a second. Put in circuit a battery, 

 a Morse key, to make the circuit when the button is 

 pressed down (another key may be used but not quite 

 so conveniently), a time-marker, and a second key, to 

 break the circuit when it is opened. B screens off the 

 Morse key, says 'Ready,' and one to four seconds later 

 presses down sharply the button of the Morse key. A 

 stands by the other key and as soon as he hears the 

 click of the Morse key opens his own key. 



Practise this a dozen times to get accustomed to it. 



Then take a tuning-fork tracing once round the drum. 

 Arrange the marker to write on* it, set the drum going 

 and repeat the experiment. The marker will mark the 

 make and break of the current, i.e. the moment of the 

 sound and the moment at which A opens the key. The 

 time between the two, the reaction time, is read off from 

 the tuning-fork tracing ; it should be rather less than Jth 

 of a second. 



2. Reaction time for cutaneous sensation. Arrange in circuit 2 

 Daniell's cells, a knock-down key (or other key breaking the circuit 

 on opening), the primary coil of an induction machine, and a time- 

 marker. Connect the secondary coil with electrodes. Connect with 

 the time-marker a key, say a Morse, and a cell. 



Place the electrodes on the moistened skin, or better on the tip 

 of the tongue, and push the secondary over the primary coil, until 

 the break shock is distinctly felt. Then proceed as in the preceding 

 section. B silently opens the (screened off) primary key. A, as 



