DETERMINATION OF WAVE FORM 649 



Cellulose nitrate made into a paste with ether and painted on 

 the tube for a distance of a few inches on each side of the plates 

 effectively prevents this trouble. 



Soft sodium glass is the best material for the tubes as it is 

 easily worked and gives the least trouble from electrostatic effects. 

 Also, the proper time during which the tube must be exhausted 

 is more readily adjusted than with other kinds of glass. 



The tube may be excited by a motor-driven electrostatic ma- 

 chine but this becomes unreliable in damp weather. In specially 

 equipped laboratories a storage bat- 

 tery of small cells, capable of giving 

 a potential of 20,000 volts, has been 

 used. A synchronous commutator, 

 Fijr. 411, has proved very service- 

 able; by it the peaks of the waves in 

 the secondary of the small high- 

 tension transformer T are rectified 

 and applied to the condenser, C, of 

 Leyden jars in parallel, around which FIG. 411. Diagram of 



the tube is shunted. Good contacts synchronous commutator 



method of exciting Braun 

 between the brushes and the com- tube. 



imitators are essential. A f high- 

 resistance rod, r, of about 100,000 ohms is connected in series 

 and directly to the cathode to prevent flash-over and to cause 

 the tube to operate more steadily. 



The intensity of the spot may be increased by using a focusing 

 coil traversed by direct current. The coil is placed axially over 

 the tube between the anode and the cathode. By this coil the 

 sensitiveness of the tube may be varied somewhat, and the vol- 

 tage necessary for the operation of the tube is reduced. Care- 

 ful regulation of the direct current is necessary. 



When working with high-frequency currents there will be, on 

 account of the inductance, large differences of potential between 

 different parts of the deflecting coils and a large electrostatic 

 deflection of the spot may occur. This may be eliminated by 

 surrounding the tube, inside the deflecting coils, by a spli 

 solenoid of fine insulated wire wound on a paper tube, 

 forms an effective electrostatic shield and has no influence on 

 the electromagnetic deflection. 



