ELECTRICITY. 



321 



practical 



Henley's 

 universal 



discharger. 



PLATE 

 CCXLIX. 

 Pi * 



Fig. 5. 



I. Henley's Univfrsnl Discharger. 



One of the most useful instruments for discharging 

 jars, was inverlted by Mr Henley, and is represented in 

 Plate CCXLIX. Fig 4. where AB is a mahogany board 

 1 1 inches long and 4 wide ; C, D two glass pillars ce- 

 mented into the base A B, and having upon their upper 

 extremities brass caps, and above them spring tubes m, 

 n, which turn round joints at o and p, that have both 

 a vertical and a horizontal motion. In these spring- 

 tubes, two wires are placed, having a ring of brass at 

 one end and a knob at the other. The small table EF, 

 about 5 inches in diameter, has a piece of ivory inlaid 

 upon its surface, and may be raised or depressed by 

 the screw nut s. The small mahogany press, repre- 

 sented in Fig. ~>. is sometimes placed in the socket * in- 

 stead of the table EF. It consists of two boards, which 

 can be pressed together by the two nuts p, q. When 

 this instrument is required to be used, the body through 

 which the discharge is to be passed, must either be laid 

 upon the table EF, or fixed in the pre.-s, Fig. 5. The 

 balls of the sliding wires must then be brought in con- 

 tact with the opposite sides of the body, and when one 

 of the rings of the wires is connected with a jar or a 

 battery, while the other ring is connected with the dis- 

 charging rod, the charge will be sent through thelxxly 

 with the greatest accuracy. In many experiments, 

 wires pointed at the extremities are used instead of 

 those which have knobs. 



2. Simple Diichargin;; Elfctromeier. 



Simple <lis- This simple instrument, which is generally attached 

 clurging to plate glass machines, is nothing more than a brass 

 Jccuouie- ball at the end of a brass stem 1 inch long, moveable 

 in a ball and socket upon the top of a glass rod fixed 

 into the base of the machine, so that the brass ball is about 

 one inch distant from the brass ball of the conductor adja- 

 cent to the piece of wood on which the receiving points 

 are fixed. The brass ball of this discharger can therefore 

 be set at any distance, within certain limits, of the con- 

 ductor, and hence it enables us to regulate the magni- 

 tude either of the sparks given by the conductor, or the 

 shock of a jar connected with the conductor. When 

 the ball of the electrometer, for example, is placed near 

 the conductor, the electricity of the conductor or of 

 the jar will always be carried off when it reaches a cer- 

 tain magnitude, proportional to the distance of the elec- 

 trometer ball from the conductor. 



3. Lant't Discharging Electrometer. 



This instrument, which is universally used, is repre- 

 hrging sented in Plate CCXLVII. Fig. 2. where fe is a piece of 

 brassfixed into the end of the conductor D. This arm car- 

 ries a bent piece of glass edc, having a brass cap cat its 

 PLATE extremity. Through a spring tube within this cap passes 

 , mi * an an " "' ' l)r "' s "Miavmga ball a, b at each extremity. 

 The arm nb is then connected with the . round by means 

 of a chain, as .shewn in Fig. 1 . where the electrometer has 

 a different position upon the conductor. When the ball 

 a M brought near the conductor 1 ). the charge of the con- 

 ductor will be carried off into the ground, by means of 

 the chain when it has reached a certain magnitude. If 

 the ball a is pushed to a greater distance from D, the 

 Conductor will require to have a higher charge before it 

 to the ball. In this way, the distance of the 

 ball a from the conductor becomes a measure of the 

 electricity which it contains at the instant of the dis- 

 . . charge. In Fig. 6. this instrument is represented as 

 VOL. vni. PART n. 



attached to a jar, but, in this case, the miter ball or ring Practical 

 must be connected with the external coating of the jar. I'-iy^"?^ 

 This electrometer is represented under a different form 



t TT J m rLATE 



at L, in Figs. 4. and 5. CCXLVII 



CHAP. II. 



On Instruments for Measuring Electricity. 



INSTRUMENTS for measuring electricity have received n ,, 

 ., ,, , n i i i On "D 



the name ot Electrometers, an appellation which has been men ts for 



given to those that indicate the presence of small quail- measuring 

 titles of electricity, as well as to those which afford an electricity. 

 exact measure of its intensity. 



SKCT. I. On Instruments that indicate small Quantities 

 of Electricity. 



\. The Abbe N-ollet's Electrometer. 



The electrometer used by the Abbe Nollet consisted The Abbe 

 of two silk threads, which indicated by their divergen- Nollet's 

 cy the presence of small quantities of electricity, and he electronic 

 even attempted to measure their magnitude by ascer- ter ' 

 taining- the angle of divergency from the shadow of the 

 threads upon a board. This electrometer was improved 

 by Mr tt'aitz, who suspended small weights to the silk 

 threads. 



2. Canton's Electrometer. 



This instrument consisted of a pair of balls turned Canton's 

 out of the dry pith of the elder. These balls were sus- electrome- 

 ]>cnded by the finest linen threads. Mr Canton kept ter. 

 them in a narrow box, with a sliding cover, and when 

 he wanted to use them, he held the box by the end of 

 the cover, and permitted the balls to hang freely from 

 a pin to which they were suspended. This instrument PLATE 

 i- reproented in Plate CCXLI il. Fig. 1 . as placed upon CCXLIH. 

 an insulating stand. 'Che balls are here shewn in a Fi B- * 

 state of divergence. 



3. Bennct's Gold Leaf Electrometer. 



A perspective view of this instrument is shewn in ,__,,. 

 Plate CCXLIX. Fig. 6. and a section of it in Fig. 7. It gold tarf 

 consists of two stripes of gold leaf w, ;/, suspended with- elcctrome- 

 m a glass cylinder ABED. This cylinder has a brass " 

 cap AB, considerably broader than itself, in the centre PLATE 

 of which is a hole a in the inside of the cap, which re- C( ' XLIX - 

 ceives a small wedge of wood. On each side of this Figs- 6> 7 ' 

 wedge two equal stripes of gold leaf, free of all ragged- 

 ness at their edges, are fixed by a little varnish : these 

 stripes are generally about two inches long, and about, 

 a quarter of an inch broad. The inside of the cap AB, 

 and the upper part of the glass cylinder, are coated with 

 sealing-wax. On the inside of the glass cylinder are 

 pasted two stripes of tinfoil b,c, diametrically opposite 

 to each other, and rising higher than the stripes of gold 

 leaf. The lower end.-, of the tinfoil are in contact with the 

 brass stand DEF, which supports the whole In order 

 to observe the electricity of the atmosphere, a pointed 

 wire C is inserted in the brass cap AB. In order to use 

 the electrometer, turn round the cap AB till the suri'aces 

 of the gold leaf are parallel to the surfaces of the pieces 

 of tinloil b.c, so that the two stripes of gold leaf may 

 hang in contact in the middle of the cylinder. Then, 

 if a body containing a small quantity of electricity be 

 brought in contact with the cap AB, the gold leaves 

 m, n will diverge, and their extremities will strike the 

 3 u 



