GALVANOTROPISM OF TADPOLES. 97 



with the same current-strength, through columns of 10 cm. 2 

 and 100 cm. 2 sectional area ; the current-density in the first 

 case is ten times what it is in the second. And the tadpole 

 accordingly is traversed by more current in the first than in 

 the second case. 



To know where we are we must therefore agree upon 

 some unit of current-density, and ascertain from measure- 

 ment of the sectional area of the water column how many 

 such units are passed through it, and — assuming the resist- 

 ance of the tadpole to be equal to that of the water 1 — 

 through the tadpole. 



We shall take as the most convenient unit of this order 

 the density afforded by 1 ampere through a column of 

 1 square metre sectional area, a value that w e may denote 

 by the symbol i«. 



You will best realise the notion of this unit by tabulating 

 a few values : — 



ampere through 1 square metre has the density ia 



0'5 2a 



O'OI IOOa 



i square centimetre, 10,000a 



milliampere, 1 100 



10 ia 



5 2a 



2 5 4a 



etc., . etc. 



The circumference of my wrist is 17 cm., i.e., the trans- 

 verse section is about 23 cm. 2 If I stand say a current of 

 5 milliamperes this implies an average current-density of 



— milliamperes per cm. 2 ^ - '- milliamperes, i.e., 2000 



23 2 3 



milliamperes per square metre, viz., 2a. 



1 The resistances are not equal, but this will not affect the dosage, 

 which will be correctly enough expressed by the number of density units 

 through the water. If the tadpole is in a fluid of lower resistance than 

 itself it will receive less than its share of current, if in a fluid of higher 

 resistance more than its share of current. This is easily verified by testing 

 a tadpole in normal saline ; when a far larger current is required to bring 

 off the effects described. 



