128 ELECTRICITY AND PROTOPLASM [Cn. VI 



saying that the current was strong or weak, others have given 

 the kind and number of elements employed. Such statements 

 are wholly inadequate to give an accurate idea of the strength 

 of current to which the organisms under experimentation were 

 subjected. (Even merely to state the galvanometer reading in 

 milliamperes is insufficient^ We must know as nearly as pos- 

 sible what strength of current is passing through the organism, 

 and this involves knowing the density of the current passing 

 through the water in the trough. Now it is obvious that a 

 current passing through a mass of water of small cross-section 

 is stronger per square millimeter than an equal current dis- 

 tributed over a large cross-section. It is necessary, conse- 

 quently, to know the cross-section of the mass of water through 

 which the stimulating current is passing, in order to determine 

 the " density " or strength at any point. For technical pur- 

 poses the unit of current-density is taken at 1 ampere to the 

 square millimeter. HERMANN and MATTHIAS ('94, p. 394) 

 propose for physiological purposes a unit one-millionth as great, 

 to be designated as S. & then indicates a current of yoVo milli- 

 ampere per square millimeter of cross-section. It is very de- 

 sirable that, when practicable, currents should hereafter be 

 expressed in 8's. More than one useless discussion has been 

 precipitated by not giving a sufficiently accurate quantitative 

 expression to the current employed. (See, for illustration, 

 below, p. 149.) 



Finally, the strength of current necessary to produce a 

 certain result depends upon the relative conductivity of the 

 organism and the surrounding water. If, through the presence 

 of substances in solution, the conductivity of the water is 

 abnormally great, one must use a greater current (as read off 

 from the -galvanometer) than otherwise to produce a certain 

 effect. (WALLER, '95, p. 97.) It would probably be best, 

 when possible, to use in the trough the water in which 

 the organism has been living, since the quantity of salts 

 in the organism has been shown to vary with that of its 

 medium. (See p. 88.)* 



* See KAISER, Wien Akad. CIV, p. 17, 1895, for a new trough adapted to the 

 stage of the microscope. 



