'94 Dr. Balth. van der Pol o)i the Production of 



a way tliat it can easily be moved in the direction AC, and 

 thus make the tubes AD slide in t\w- tubes DC. A pointer 

 connected to the support of A and running over a scale in- 

 dicates the total length AC of the wires. 



The mercury cups and the hole in the middle of A are 

 intended for the insertion of small condensers at the end of 

 the Lecher system, e. g. for the determination of specific 

 inductive capacities of liquids. 



For the measurement of the current amplitude in the 

 Lecher system we used a Duddell thermo-galvanometer. In 

 order to find a firm support for it the latter had to be placed 

 several metres away from the centre of the Lecher wires. 

 The terminals of the galvanometer were connected to a pair 

 of insulated copper wires, the last part of which ran vertically 

 and ended freely just above the Lecher wires somewhere at 

 \ of the wave-length distance B from the ends. The distance 

 -of the free ends of these galvanometer leads from the 

 Lecher wires was made variable from about 1 mm. to a few 

 centimetres. To this end they were connected with sealing- 

 wax to a piece of horizontal glass tubing supported by an 

 ordinary stand placed some distance away from the Lecher 

 wires. By lifting or lowering this glass tube, or by moving- 

 it in the horizontal direction perpendicular to the Lecher 

 wires (so that the ends of the galvanometer leads were no 

 longer symmetrically placed in the electric field of the 

 Lecher system), the deflexion of the galvanometer could for 

 a constant wave-amplitude be reduced or increased many 

 times. As might have been expected, a 377 ohm "heater" 

 in the galvanometer gave under similar conditions greater 

 deflexions than one of 3'8 ohm, though the latter heater 

 resistance gives ample deflexions for most purposes. When 

 the Lecher wires are supported in a potential node (for 

 the fundamental the point D, where DC= J\) ? the distance 

 between the ends of the galvanometer leads and the Lecher 

 system will not vary when the tubes AD are moved in DC, 

 and no bending of DC is likely in this way. If there are 

 indications that the galvanometer leads have a natural 

 frequency near the one to be measured, they can easily 

 be brought out of tune by simply placing a short piece 

 of wire across the insulation of the horizontal part of 

 the galvanometer leads, and shifting it over them till 

 all disturbing resonance of the galvanometer leads is 

 eliminated . 



In this way we obtained, by varying the length AD, a 

 resonance curve with its first very sharp maximum w r hen 

 .AC — ^\ and a second maximum when A was further 



