586 



NATURE 



{Oct, lo, 1889 



surrounded by an atmosphere of positively electrified giseoas 

 particles extending to the outer ed^e of the sD-ciUed dirk spice. 

 According to the author's views, this atmosphere corresponds to 

 the polarized layer adjoining the negative electrode in an electro- 

 lyte. The cause of the sudden difference in luminosity between 

 the dark space and the negative glow has also been investigated, 

 and it has been found that negative particles projected froai the 

 kathode pass unhindered through the dirk spice, while their 

 velocity is quickly reduced in the glow proper, the translatory 

 energy being thus changed into energy of vibration. 



Prof. Oliver J. Lodge, F.R. S., read a paper on the failure 

 of metal sheets to screen off the electrostatic effect of moving or 

 varying charges. Experiments have been made on the screening 

 effect of a very thin film of silver chemically deposited, the 

 thickness of the deposit being different in the different experi- 

 ments. The silver screens are only found to protect so long as 

 they are opaque ; they no longer do so when the deposit is 

 so thin as to be transparent. 



Prof. James Blyth described a new form of current-weigher. 

 In the construction of balances for the measurement of electric 

 currents, a greater or less difficulty has always been experienced 

 in leading the current into the movable parts of the instrument 

 without seriously interfering with its sensibility. Several methods 

 have been adopted to overcome this difficulty. In some balances 

 the current is led in by mercury cups ; in others, flexible leads, 

 made of thin wire spirals or thin metal strips, are employed ; 

 while, in the recent balances by Sir William Thomson, the diffi- 

 culty has been overcome by means of his well-known ligature 

 suspension. Some time ago it occurred to the author that still 

 another form of balance might be employed for this purpose, 

 and the present paper is a short description of one which he has 

 made. The balance referred to is of the ordinary Roberval 

 type, with the pivot connections all replaced by tightly-stretched 

 torsion wires. It is constructed as follows : — On a flat base- 

 board are placed two vertical uprights of wood or other insulat- 

 ing material, about 6 inches apart. Between these are stretched 

 two parallel wires in the same vertical plane about 3 inches 

 apart. To the middle points of these wires are soldered the two 

 horizontal metal bars of the Roberval. These are about 9 inches 

 long. Both horizontal bars terminate at each end either in forks 

 or rings, placed in a horizontal plane, and wires are tightly 

 stretched between the prongs of the forks, or across a diameter 

 of the rings. To the middle points of these last wires are 

 attached, also by soldering, the vertical bars of the balance, 

 thus completing what takes the place of the ordinary jointed 

 parallelogram of the Roberval. The vertical bars have metal 

 terminals, insulated from each other, and carry the circular 

 disks, on the rims of which the movable coils of wire are wound. 

 The bars pass at right angles through the centres of the disks, 

 and are fixed to them at their middle points. The middle coils, 

 which are of exactly the same diameter as the movable ones, are 

 supported from the base-board, and are placed so that one is 

 about half an inch above one movable coil, while the other 

 is as much below the other movable coil. From this it 

 will be seen that, when the balance is in equilibrium, the 

 fixed and movable coils are all horizontal, with a space 

 of about half an inch between each pair. The stretched 

 wires may be either of steel or phosphor-bronze, and before 

 being finally soldered are placed under considerable tension. 

 The current flows through the instrument thus : — Entering, say, 

 by the upper wire connecting the fixed supports, it passes to the 

 upper horizontal bar. There it splits into two, one half (sup- 

 posing the resistances equal) passing to each end of the bar, and, 

 by means of the corresponding fork-wires, passing through the 

 movable coils. From the movable coils each half returns along 

 the lower horizontal bar, and together pass out by the lower 

 wire connecting the two main supports. From this the whole 

 current passes first through the one fixed coil, and then through 

 the other, and in such a direction as to produce a repulsion 

 between each pair of coils. In the constructing, care is taken 

 that the suspended coils are both made of equal weight, and 

 that when the balance is in equilibrium no torsion is in any of 

 the wires. Small scale-pans are attached to each vertical bar, 

 and a bob for raising or lowering the centre of gravity of the 

 whole is placed on a rod springing at right angles from the 

 middle of one of the horizontal bars. Tne current strength is 

 estimated by the weight needed to restore the balance to eqiili- 

 brium when disturbed by the passage of the current. A sliding 

 weight may also be used, as in a steelyard. It will be readily 

 seen that, as in all form; of carrent- weigher, the weigits are 

 proportional to the square of the current strength. 



Prof. S. P. Thompson described a phenomenon in the electro- 

 chemical solution of metals — originally discovered by Plante — 

 which occurs when a current is passed between two copper wires 

 immersed in dilute sulphuric acid in a cell. The solution takes 

 place in two stages, the metal first oxidizing, and then the oxide 

 dissolving in the acid. This was projected on the screen, the 

 audience being able to observe that the bubbles evolved when 

 the current was turned on ceased as the current died down, 

 choked by the formation of oxide, but almost immediately re- 

 commenced when the oxide began to dissolve. The author has 

 tried other metals and other acids — silver, zinc, and iron showing 

 similar effects in sulphuric acid, but not in nitric, acetic, or 

 hydrochloric acids. 



Mr. J. Wilson Swan read a paper on chromic acid as a depolarizer 

 in Bunsen's battery. When chromic acid became an article of 

 commerce at a moderate price, it occurred to the author to see if 

 he could not obtain a substitute for the nitric acid of the Bunsen 

 battery. As the results of his experiments he finds that a 

 solution, of the composition nitric acid i part, chromic acid 2 

 parts, sulphuric acid 5 parts, and water 5 parts, gives results 

 equal to that obtained with nitric acid. 



Prof. Perry, F.R S., described a variable standard of self- 

 induction. An instrument like that hitherto used as a variable 

 standard by Prof. Ayrton and the author, was used by Prof. 

 Hughes. It is a fixed coil of wire, inside which a movable coil is 

 placed ; the coils are in series one with another. When their 

 planes are parallel there is either a minimum or a maximum co- 

 efficient. When the movable coil is rotated so that its plane 

 makes various angles with the plane of the fixed coil, a pointer 

 shows on a scale the coefficient for that particular position. 



In another paper on a hot twisted strip voltmeter, Prof Perry 

 described the behaviour of twisted strips subjected to axial pull. 

 A small elongation is accompanied by a great rotation, so that 

 these strips may be employed in measuring- instruments such as 

 weighing-machines, aneroid barometers, testing-machines, &c. 

 For the Ayrton and Perry voltmeter a double twisted strip of 

 platinum silver with its ends insulated is initially in tension inside 

 a tube or frame two-thirds brass and one-third iron, a pointer at 

 the middle of the strip being visible above a dial and capable of a 

 motion of nearly 360°. When the current passes the pointer 

 rotates because of the heating of the strip. By continuous making 

 and breaking of a large current through the strip during seventy- 

 two hours all zero and other errors seem to be eliminated. The 

 highest reading of the exhil)ited instrument was 2^ volts, so that 

 it is particularly useful in accumulator-worlv. The author gave 

 the law of transformers generally, and found that a small trans- 

 former on the base of the instrument converted it into a voltmeter 

 of any range whatsoever for alternating currents, the readings 

 being mdependent of the frequency of alternation. The author 

 exhibited twisted strips of carbon made by Messrs. Woodhouse 

 and Kawson, which he intends to use for voltmeters of higher 

 range. 



Mr. W. H. Preece, F.R. S. , r,>adapaper on the relative effects 

 of steady and alternate curre ts on different conductors. Sir W. 

 I'houison, at the Bath meeting of the British Association, stated 

 that alternating currents entered a distance of only about 3 

 millimetres into the heart of a thick round copper conductor, 

 when the frequency was 150. This "diffusion law," as he called 

 it, is dependent on the coefficient of self-induction and on the 

 frequency, i.e. on the number of complete alternations of positive 

 and negative currents transmitted per second. As this law has a 

 most important bearing on the commercial value of systems of dis- 

 tribution dependent on alternating currents, it becomes most 

 desirable to study the question practically. The verification of 

 this law is almost beyond the reach of experiment. It occurred 

 to the author, however, that if conductors of different materials 

 are taken, such as iron, copper, lead, and platinoid, of easily 

 measuralile lengths and of convenient sectional areas, and if 

 measurable and variable currents, both alternating and direct, 

 approximately similar to those used in practice, are transmitted 

 through them, the question could be studied by observing any 

 dififc-rence, if such existed, in the total expenditure of energy 

 in the conductors under these different circumstances. The 

 general conclusion to be drawn from the experiments is that 

 practically no serious error has been made in the form of con- 

 ductors so much used for alternating current systems, and that 

 nothing cheaper or better has been devised than a simple stranded 

 conituctor coated with a suitable insulating coating. The experi- 

 ment- do not solve the question of the distribution of current 

 densjiy through the section of the conductor, but they do 

 show that within the range of practice the total flow of energy is 



