April 19, 1894] 



NA TURE 



591 



Brown-Sequard and M. C. Paul were the earliest to explain the 

 therapeutic value, would find a place in materia medica, nor 

 cultures of microbes. It was not foreseen that we should have 

 to chronicle in 1894 the sale not only of sequardine, but also of 

 veritable bacterial products such as tuberculine, tuberculocidine, 

 antiluberculine, antitoxine, k.t.A. How shall we check the 

 fury of this flood ? There seems no reason why it should ever 

 come to an end. 



TRANSPARENT CONDUCTING SCREENS FOR 

 ELECTRIC AND OTHER APPARATUS.^ 



TT is well known that electrostatic instruments require to be 

 screened from outside electric disturbance, in order that 

 their indications may be correct ; but it is not so generally 

 recognised that instruments intended to measure small forces, 

 such as certain types of electro- magnetic voltmeters, delicate 

 vacuum gauges, &c. are liable to give wrong readings from an 

 electric attraction being exerted on the pointer, such as is 

 produced by the glass cover when it is touched or cleaned. 



There is on the table here a well-known type of gravity 

 electio-viagnetic voltmeter, which may be found on the switch- 

 boards of many English and continental electric light stations. 

 At the present moment its terminals are not connected with the 

 electric light mains of the building, so that it should indicate 

 zero pressure. Let me, however, but stroke the right-hand side 

 of the glass cover with my finger, and the pointer, as you see, 

 at once turns to eighty volts or more. Conversely, let the 

 terminals of the voltmeter be connected with the electric light 

 mains ; the pointer should point to about 100 volts, for that, as 

 you know, is the pressure supplied by the Westminster Com- 

 pany. The voltmeter appears to be indicating correctly, but, 

 on stroking the left-hand side of the glass cover, the pressure, 

 as read by the instrument, appears to suddenly fall to some 

 forty volts. And a similiar effect is produced if a piece of 

 wash-leather or dry waste be used in place of the finger. 



If, then, it is possible to cause this instrument to indicate at 

 will sixty or eighty volts too high or too low, how impossible 

 must it be to feel sure that the glass cover — which is, of course, 

 maintained in a dry condition in a hot engine room — has not 

 been electrified by some accidental touch of the coat sleeve 

 sufficiently to cause an error of three or four per cent, in the 

 reading of this voltmeter ! 



We find that it is not merely with this particular type of 

 voltmeter that an error can be produced by stroking or rubbing 

 the glass cover, for other electro-magnetic instruments that we 

 have tried can also have their pointers deflected^^in the same 

 way, but not to the same extent. 



Nor, of course, is this source of error in any way connected 

 with a voltmeter being an instrument constructed to measure an 

 electrical magnitude, for it would equally exist if the glass were 

 clean and dry and the controlling force remained of the same 

 magnitude, no matter what was the quantity the instrument was 

 constructed to measure. For example, on the table there is a 

 vacuum gauge the wheel-sector-pinion of which has been re- 

 placed with an Ayrton-Perry magnifying spring. This gauge 

 is, no doubt, very sensitive, for you observe that the pointer 

 moves even when I produce an extremely slight diminution of 

 pressure by rotating the short length of india-rubber tube as 

 slowly as I can ; th ; change of pressure on pinching the tube, or 

 even on dropping it, is indicated by the pointer. On the other 

 hand, the pointer is of glass, and therefore is not suitable for 

 being acted on by an electrostatic force ; still, a stroke on the 

 glass cover, as you see, causes the pointer to deflect through 

 several degrees. 



It has been known for a long time that it is possible to screen 

 an instrument from such outside electrostatic disturbances by 

 surrounding it with a metallic cage composed of wire or of 

 strips of tinfoil. Such a method of screening, however, has the 

 great disadvantage that it renders it difficult to observe the 

 exact position of the pointer from a distance, for the wires or 

 strips of tinfoil cover up the pointer more or less. We there- 

 fore thought of placing the pointer underneath the metallic 

 dial of our electrostatic voltmeters, and of only allowing the 

 tip to project through a slot in the dial plate. But this method 

 we abandoned on trying it eighteen months ago, 'for to make the 



1 A paper by W. E. Ayrton, F.R.S., and T. Matlier, read at the 

 Institution of Electrical Engineers on April 12. 



NO. 1277, VOL. 49] 



screening good the visible part of the pointer must be reduced 

 to a spot, and the exact posifiofi of this spot we found less easy 

 to read at a distance of several feet than that of a long black 

 line, which is the appearance of a pointer when it is visil)le 

 along its whole length. This method of screening has, how- 

 ever, we understand, been recently adopted by a firm of 

 instrument makers. 



We next considered whether it was not possible to make a 

 perfectly transparent conducting screen, so that, while the elec- 

 trostatic screening of the pointer should be practically perfect, 

 the pointer and dial should be as easily seen as if the screen 

 were not present. Our first idea was to make the glass cover 

 double, and to insert between the two sheets of glass a layer of 

 clear conducting liquid. Fearing, however, trouble from leak- 

 age of the liquid, or from the liquid becoming gradually turbid 

 and giving the dial a dirty appearance, v/e turned our attention 

 to depositing films of solid matter on the inside of the glass 

 cover, or shade, of sufficient thinness to be practically trans- 

 parent, but with the solid particles near enough together to be 

 conducting. We tried smoke, silver deposited in layers of 

 various thicknesses, mercury vaporised and deposited, sal- 

 ammoniac vaporised and deposited, &c., but we were quite 

 unable to obtain in this way both transparency and electric 

 conduction. 



After a conversation with Prof. Boys, when discussing the 

 problem that we were then engaged in solving, we commenced 

 experimenting on varnishes, with the view of arriving at a 

 varnish which should be as hard and as transparent as cleay 

 shellac, but which, instead of being an insulator like shellac, 

 should be a sufficiently good conductor to allow of the instan- 

 taneous production of an induced electric charge to balance the 

 electrostatic action of any outside body. Glass plates were 

 coated with gum, with coaguline, with the gelatinous electrolyte 

 used in accumulators (composed of sodium silicate and dilute 

 sulphuric acid), with isinglass dissolved in acetic acid, with 

 gelatine dissolved in acetic acid, with isinglass dissolved in a 

 mixture of acetic and sulphuric acids, and with gelatine 

 dissolved in the same mixture. After much experimenting, we 

 arrived at the following two methods of coating a glass cover, 

 or shade, which gives perfectly satisfactory results : — 



No. I. — Dissolve \ ounce of transparent gelatine in i ounce 

 of glacial acetic acid by healing them together in a water bath 

 at 100° C. To this solution add half the volume of dilute 

 sulphuric acid which has been prepared by mixing i part of 

 strong acid with 8 of distilled water by volume, an I apply the 

 mixture while still warm to the glass shade, which should be 

 previously polished and be warm. When this film has become 

 very nearly hard, apply over it a coating of Griffith's anti- 

 sulphuric enamel. 



Method No. 2. — -Thin the gelatine solution, prepared in the 

 manner previously described, by the addition of acetic acid (say 

 2 volumes of acid to i of the solution), and, after polishing the 

 glass, float this thinned solution over the glass cold. Drive off 

 the excess of acetic acid by warming, allow the glass to cool, and 

 repeat the fl lating process, say, twice. Thin the anti-sulphuric 

 enamel by the addition of ether, and float it over the gelatine 

 layer applied as just described. Expel the ether by heating, and 

 apply a second layer of this thinned anti-sulphuric enamel. 



With experience, such as Messrs. Elliott and Messrs. Paul 

 have at length acquired after much practice, a layer can be 

 applied, either according to method No. i or No. 2, so that, 

 when finished, it is quite hard to the touch, and so transparent 

 that it is only by looking at the glass plate oWliquely that the 

 presence of the varnish can be detected. It is also so conducting 

 that when a P.D. of several thousand volts, alternating with a 

 frequency of 200, is set up between the needle and inductors of 

 one of our electrostatic voltmeters, the pointer, which is metal- 

 lically part of the needle, is not visibly attracted by a metallic rod 

 held just outside the glass close to the pointer, this metallic rod 

 being electrically connected with the stationary inductors. 



Without experience, however, it is somewhat difficult to 

 apply the coating so that it is not either cloudy, or a com- 

 paratively poor electrostatic screen, or both. 



This second electro-magnetic voltmeter — which, like the former, 

 has been kindly lent us by Mr. Barley, of the Knightsbridge 

 electric light central station — looks exactly like the other one, and, 

 indeed, behaved exactly like the other one when we received it. 

 It has, however, had a layer of our transparent varnish applied on 

 the inner side of the glass subsequently, and you will find that 

 you may rub the glass as much as you like, or even hold a rubbed 



