300 



A. TANAKADATE 



niimher of layers, it will be easy to make the above value 1 volt or 

 less. Indeed this would have been done in the instrument now l)erng 

 described had it not been at the time /impossible to obtain in tliis 

 country a tine enough insulated copper wire. No doubt, the value 

 required could be arrived at by simply reducing the resistance to little 

 more than 1000 ohms. This however would be too small for the 

 purpose; if possible 10,000* ohms or more would be desirable. 



§ IV. External Coil. 



A single rectangular coil consisting of several hundred turns of 

 a tine copper wire ( see Fig. 12.) may take the place of this internal 

 coil just described, if one part of the coil be temporarily slipped in 

 between the jaw in the same M^ay as the so called ribbon was slipped 

 in when the irstrument was being gauged. In simplicity of construc- 

 tion, this plan is perhaps superior to that of introducing the internal 



Fiq. 12. 



coil especially wlien the instrument i.-^ to be made on a small scale, 

 lînt it is inftirior inasmuch as it needs an extra piece of apparatus. 



* Thomson's f^radeil potential Kalvauoinetor has ahoiit 7ü!)0 ohius. 



