418 



RECENT PROGRESS IN PHYSICS. 



quotients and taking their mean, we iind the value 1.97 deviating 

 but little from 2. 



In like manner comparing the third, fourth, and fifth vertical 

 series of the values of q with the first, we get as mean values the 

 quotient : 



3.05 3.84 4.94 



instead of 



3. 4. 5. 



Thus it is seen that the mean values agree quite well with the law. 



It has already been proved by Coulomb's experiments that two 

 insulated conductors which are in contact, after receiving electrical 

 charges, repel each other with a force proportional to the square of 

 the electrical density. 



In the experiments just described we do not directly measure the 

 density of E upon the balls, but the quantity induced upon the outer 

 surface of the battery. The accordance of our results with Coulomb's 

 law therefore proves that the density of the free E of the inner coat- 

 ing, producing the repulsion of the knobs, is always in the same 

 proportion to the induced E on the outer coating ; or, in other words, 

 that the co-efficient of condensation is independent of the quantity of 

 E in the interior of the battery. 



§ 31. Striking distance of the battery — The experiments of Eiess on 

 this subject (Pogg. Ann. XL, 332) confirm the fact, which had been 

 already discovered by Lane and iZarn's, that the striking distance of 

 the battery is proportional to the density of the electricity. 



In order to measure accurately the striking distance of the battery, 

 Riess used an apparatus, which he termed the spark micrometer ^ rep- 

 resented in figure 50. 



Fig. 50. 



Each of the brass 

 pins, « and h, is at- 

 tached to a piece of 

 brass having a horizon- 

 tal arm for clamping- 

 wire and insulated by 

 a glass support. One 

 of the rods is fixed, the 

 other is on a slide which 

 moves by means of the 

 screw / along a gradu- 

 ated scale. When the 

 clamp screw d is loose, 

 the slide may be moved 

 freely by the hand; but 

 when d is screwed up, 

 the fine adjustment is 

 made by means of /, 

 because by screwing up 

 d the nut belonging to 

 / is clamped against 

 the lower metal plate. 

 The whole apparatus rests upon a glass support 2^ inches high. 



