Wave-length of Electric Radiation by Diffraction Grating. 17 i 



narrow slits, through which the pin projects. The slits allow the 

 necessary sliding for placing the radiator and the receiver on the 

 focal curve. It would be better to have the sliding arrangement at 

 the free ends of the arms, the pin passing through the central ends, 

 acting as a pivot. The circle is graduated into degrees, but one- 

 fourth of a degree may be estimated. 



Description of the Apparatus. 



The Radiator. Electric oscillation is produced between two 

 metallic beads and an interposed sphere 0'78 cm. in diameter. The 

 beads and the interposed sphere were at first thickly coated with 

 gold, and the surface highly polished. This worked satisfactorily 

 for a time, but, after long -continued action, the surface of the ball 

 became roughened, and the discharge ceased to be oscillatory. 

 After some difficulty in obtaining the requisite high temperature, I 

 succeeded in casting a solid ball and two beads of platinum. There 

 is now no difficulty in obtaining an oscillatory discharge, and the ball 

 does not require so mnch looking after. 



As an electric generator, I at first used a small Ruhmkorff's coil, 

 actuated by a battery. I, however, soon found that the usual 

 vibrating arrangement is a source of trouble ; the contact points soon 

 get worn out, and the break becomes irregular. The oscillation pro- 

 duced by a single break is quite sufficient for a single experiment, 

 and it is a mere waste to have a series of useless oscillations. But the 

 most serious objection to the continuous production of secondary 

 sparks is the deteriorating action on the spark balls. Anyone who 

 has tried to obtain an oscillatory discharge knows how easily the 

 discharge becomes irregular, and the most fruitful source of trouble 

 is often traced to the disintegration of the sparking surface. In my 

 later apparatus I have discarded the use of the vibrating interrupter. 

 The coil has also been somewhat modified. A long strip of paraffined 

 paper is taken, and tinfoil pasted on opposite sides ; this long roll is 

 wound round the secondary to act as a condenser, and appropriate 

 connexions made with the interrupting key. This arrangement 



Fm. 3. 



The Radiator. 



VOL. LI. 



