( 251 ) 



string 10 : 1 quart/ to 3.5 silver 



,, lo ,, 1 5, „ 0.4 ,, 



14 1 2 4 



We now proceed to express the resistance to the motion of the 

 string in absolute measure. According to the definition formerly given 

 r is the virtual resistance to the motion of the string in micram- 

 peres, when the image of the string moves with a velocity of 1 mm 

 distance per 1 mm time. 



We call r' the resistance to the motion of the string in dynes when 

 the middle of the string moves with a velocity of 1 cM. per second. 



The above mentioned unit r refers to a strength of field H, a 



length of the quartz thread /, a magnification b and a speed of the 



writing plane V. 



HI 

 Since the force of 1 ^lA is equal to — - dynes, we may write : 



HI 10 h 

 ^ 107 ^ F 

 or 



r' = rX-^ (34) 



-^ 10f>F ^ 



Substituting in this the above values for H, I, h and V, we get 



/ = 0,295 7- (35) 



It is unnecessary to give here the absolute measures of the elec- 

 tromagnetic damping. These were discussed in chapter IV where 

 they served us for accurately determining the value of H. 



On the other hand the absolute measures of the air-damping r'a 

 may find a place here. 



In chapter IV the air-damping was found 



for string N". 10 Va = 0,0193 [mm - iiA] 

 „ „13 r„ = 0,0174 „ 

 „ „ 14 Va = 0,0157 „ 

 By formula (35) we calculate from this 



for string N°. 10 r'a = 0,00569 dynes. 

 „ „ 13 r'„=: 0,00513 „ 

 „ „ 14 r'a = 0,00463 „ 

 It would be desirable to compare these values with those that 

 might be calculated by means of the kinetic theory of gases. But 

 then we ought to bear in mind that we have combined in Va other 

 causes of damping besides the air-damping. 

 These causes are threefold : 



