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XXXT. A Method of Measuring without Electrodes the Con- 

 ductivity at various points alone/ a Glow Discharge and 

 in Flames. By Balth. van der Pol, Jun., Docts. Sc. 

 ( Utrecht) *. 



WHEN stationary waves are induced in a pair of parallel 

 wires tuned to the frequency of a generator, the 

 current in this Lecher system will increase till a stage is 

 reached where the energy supply from the exciter is just 

 balanced by the energy dissipation in the wires. 



The principal part of this dissipation is to be found in the 

 Joule losses, while the radiation from the Lecher system will 

 in general be very small, due to the currents in opposite 

 parts in the two wires being 180° out of phase. But when 

 at a point in the field between the wires some small body is 

 introduced having a finite conductivit} r , energy will be dis- 

 sipated in it as well. Under a constant excitation the 

 current (and Potential Difference) will therefore in this 

 case only reach a lower final value, and this latter current 

 limit is a measure of the energy dissipation in the substance 

 brought between the Lecher wires. It is, however, not a 

 unique measure for the conductivity a of the body con- 

 sidered, as is obvious from the consideration that both 

 when 



o- = and cr = oo 



no absorption will take place. Hence a second factor must 

 come in, and this is the alteration in the period caused by 

 the presence of the hody in the electric field. 



The above considerations are further equally valid whether 

 the body brought in the field between the wires is electrically 

 connected to them or whether it is not. 



The procedure is therefore of value when the absorption 

 in a substance is to be measured where the presence of 

 separate electrodes would either disturb the natural state 

 of affairs (as is often the case with test electrodes in a glow- 

 discharge) or where a strong polarization at the cathode 

 makes the direct-current electrode measurements complicated 

 or uncertain (as in flames). 



For mathematical treatment the simplest way is to con- 

 sider the body to be located in one single point. 



As our experiments were confined to the body being placed 

 near the free ends of the Lecher wires, where a potential loop 

 occurs, we will only consider here this case. When the body 



* Communicated by Sir J. J. Thomson, O.M., P.R.S. 



