Ji54 



another somewhat larger capacity L\ and without inodifviiig T but 

 hv putting a resistance W into the circuit we again obtain a minimal 

 contraction. 

 We then put : 



or 



-= H'-^^T ........ (3) 



We mav take for C', any capacity larger than t\. By taking 



1 

 C\ = 2C'i we get the extremely simple expression for - : 



^^ 



\=^C, ......... (4) 



or the chronaxia is the product of the resistance II' and thecapacity C\. 



If we can use a complete set of graduated condensors we might 

 use a tixed resistance W and try to tind the value of C\ giving a 

 minimal contraction, using formida (3). But as a niatter of ftict we 

 find the use of a calibrated rheostat more convenient, especially as 

 it allows of the use of formula (4), which is sim|)ler. In this case 

 only two condensors of say 0.05 or 0.1 ^iF and a rheostat up to 

 10000 Ohm aie necessary; wiieieas in the first method a set of 

 condensors from 001—0.5 [iF and a fixed resistance W of 1000 

 or 2000 Ohm would be required. 



The actual measurement of the Voltage V is unnecessary. 



Induction coil method. 



The secondary discharge of a meiiical iniluction coil may be 

 represented by 



M ^^t 



/,,z=r/(0=:7, — f / n (5) 



in which I^ is the primary current, i\f the mutual induction coeffi- 

 cient, Lji the selfinduction and i?i, the resistance of the secondary 

 circuit. 



Putting (f {i) in Hoorweg's formula (1) we get 



^i=:ttl^ — f ^z-.i '^ <n = - '-— - 







and taking ^i = 1 we get -. 



al M=:R,,+ i3L,, ....... (6) 



We now make again two measurements of a minimal contraction. 

 In the first one we insert a selfinduction L into the secondary 



