A.M.Mayer — A large Spring-balanc< Electrometer. 517 



the earth-plate and disc by / — But as in the two experi- 



ments the force on the mica-disc is equal, we must have caused 

 an equal decrea* in capacity by increasing the distance be- 

 tween earth-plate and disc by the quantity * — *, when the 



dielectric intervened, and this measured increase of distance 

 equals the imaginary decrease of distance produced by the 



dielectric, and we have t = —(s —s) and K= — , r.* 



K ' ' t — (s i ^s) 



We use a disc of sulphur about 2i cms. thick as the body 

 to experiment on in this measure before a class, as sulphur is 

 very slightly, if at all, affected by time-induction during the 

 interval occupied by the measure. 



If the guard-plate be removed the phenomena become more 

 apparent to the audience, but the measure is not so exact. 

 In making measures without the guard-plate we use the tele- 

 pe and the line on the glass tube, or, for ordinary qualita- 

 tive illustration a pointer may replace these. 



It need hardly be stated that measurements with this appa- 

 ratus are of little value when compared with the measures made 

 with the excellent apparatus of Gordon where entire control 

 of the time of charging and means of measuring, very minute 

 differences of capacity are used. Nevertheless the measures 

 obtained with this apparatus are really more reliable than 

 those obtained by some of the older experiments. The exhi- 

 bition of the actions of this apparatus, however, are well 

 worthy of being shown to students. The clear ideas and last- 

 ing impressions they give, from their directness and easy com- 

 prehension, will reward the teacher for the time he may give 

 to the construction and use of the apparatus. 



The same instrument will also measure potential, for it is 

 really a huge and rough absolute electrometer. Certain pre- 

 cautions are, however, needed in its use in such measures. In 

 the formula? used in measurements with the absolute electrom- 



eter, V= D v/^J^, orV-V'=(D-D')V^5^, one 



might suppose that W could be determined for the spring- 

 balance electrometer by merely adding the weight in grams 

 required to depress the silvered disc down to the level of the 

 guard-plate ; but, on electrifying the helical spring, its coils 

 separate by mutual repulsion and a certain lengthening of the 

 spring results. This action may be determined and allowed 

 for by first ascertaining the weight of mica-disc and its wires 

 and additional weight required to depress the disc to contact 

 with the wires of guard-plate. The disc is now detached from 



* See Gordon's Electricity and Mag., vol. i, p. 111. 



