72 



SCIENCE PROGRESS 



body. In order to record the rate at which the fields appear 

 and disappear, we select some spot in each, e.g. the mouth 

 and one of the hind legs, and with some good conductor of 

 electricity (such as wool or thread soaked in salt water) connect 

 each with a basin of salt water, these in their turn being 

 connected with the terminals of an instrument sensitive enough 

 to record such small differences of potential as come into existence 

 between the two fields. Such an instrument is the capillary 

 electrometer represented diagrammatically in fig. 2, which shows 

 a bird ready to have its pulse taken. The instrument consists 



^ 



I 



H,SO, 



Hg 



Inverted image of 



tip of capillary 



X 300 



H.,S04 



Fig. 2. — Diagram of a bird having its pulse taken. 



essentially of a fine glass tube, drawn out so as to be only a few 

 thousandths of a millimetre in diameter near the tip, filled 

 with mercury. The tip dips into dilute sulphuric acid, which 

 enters so far as the mercury permits, the tube being very slightly 

 conical so as just to prevent the mercury running out however 

 near it be to the tip. The properties of the instrument are such 

 that if the mercury becomes (galvanometrically) positive to the 

 acid it moves towards it, if negative it moves in the opposite 

 direction. Since the one field always comes into existence 

 before the other, even though it may be by no more than a 

 thousandth of a second, there is always a quick movement of 

 the mercury in one direction while the single field exists. 



