Farther experiments led to a " thistle tube" of glass arrangement, the tube (Fig. 98) being bent in a gas Same as shown, mercury placed therein, and copper wire being passed down the stem into the mercury. This is simply-placed in a jar of solution containing the carbon, but the mercury must receive occasional replenishmenta of small pieces of sheet zinc, which readily dissolve in it. The thistle tube is convenient for lifting in and out, and it can be lashed to a spring clothes peg clipped on the top of the jar.
But for permanent use a small shallow pot of earthenware (such as Needham s metal polish is sold in - Fig. 99) has a round zinc rod cemented in firmly, so as to form a means of lifting in a complete state out of the outer battery jar. Mercury is then poured In to form a "water joint," a paraffin lamp chimney with a "crinkled" top (Fig. 99) is then Inverted and passed down the zice rod into the mercury, and filled with pure water to counterbalance the pressure of the surrounding fluid (bichr. potass + ac. sulph., usual solution), in which stands the carbon.
This water remains purely free from any acidity for months, and by careful tests I And that the addition of sulphuric acid simply weakens the battery by setting up a "counter" current. In the "porous pot form" the acid seems quite necessary to lower the resistance of the fluid, but here it is different. Of course a preferable form to avoid risk of cement giving way would be to make the glass and the dish in one (in glass), with piercings to ensure the continuity of the body of mercury in contact with the zinc and the bichrora. ac. fluid.
The thistle tube arrangement I do not like, owing to having to supply the zinc, and the surface of mercury is too small, but the other arrangements I have tested for six months successfully. (Elec. Review)-
Selenium- - Fritta and Hopkinson have devised a new process for manufacturing very sensitive selenium elements in which the entire mass is influenced by light. This result is obtained by a preparation of the selenium itself, and the construction of elements whose sidea are made of a material that conducts both electricity and light well. Several thin sheets of reheated selenium are enclosed between these sides, and it results therefrom that the current traverses the selenium in the direction of the light that strikes it; and it appears that, owing to this arrangement, the change* in resistance caused by the light (or, in other words, the property that selenium possesses of regulating light) become much developed.
The selenium is selected in as pure a state as possible, and is formed into sheets that are placed between blocks of a material to which it does not adhere. The selenium is softened by heat, so as to render it as thin as is judged necessary, and the sheets are afterward cooled under pressure. Two thin sheets of mica are introduced between the selenium and the blocks, so as to facilitate the separation of the sheets of selenium and the blocks forming the mould. When it is desired to have sheets very sensitive to the light, it is necessary to make them so thin that they appear, before reheating, of a blood-red colour whoa they are looked at against the light.
Fig, 100 shows the press employed fur softening the selenium plates by this process. It consists of a heating box a, with a door o', and a strong shelf 6, provided with a groove in which slides the piece V. A screw c serves to exert pressure, and c' represents a support which passes along the sides of the box and over the shelf 6. A gas flame d furnishes the heat, and a thermometer d' shows the temperature. The apparatus is completed by a certain quantity of scrap iron, which equalises the temperature under the pressure plates b2, b2. These latter are placed upon a movable piece o', and their position is so regnlated as to make their centres coincide with the .aiia of the screw c, which then exerts an equal pressure. By this process the selenium is softened and converted into sheets of the desired thickness. Id order to make elements of these sheets, one of them is filed (Fig. 101) in the centre of a. rubier frame e, provided on each side with metallic supports f f, and the sheet is insulated from the latter by bands of a proper material. On each side is a glass cover g g', filed to the rubber frame e by cement. The supports f are connected with the wires f2 f3 and communicate thereby with the pile or the circuit of the electric current.
The two sides of the sheet of selenium dip into a transparent conducting liquid, which is poured into the elements by means of tubes e e', that likewise serve for the eiit of the gases formed through the electrolysis of the liquid. The light traverses the liquid and falls upon the selenium plate of the element, and the current reaches the plate by the same route. Sometimes wires placed at equal distances are made to pass through the liquid, in order to secure an equal distribution of the current in the liquid electrode. These wires are stretched between the metallic supports f f1. Moreover, movable shutters are sometimes placed upon the glass sides of the element, so that the light that strikes them may he regulated.
Instead of a liquid condoctor, sheet platinum, gold, or silver is sometimes used as the electrode of the selenium, but this must be thin enough to allow the light to pass through it. Gold leaf answers very well, and, when used, is first wetted with alcohol, and the selenium is then placed over It, and afterward reversed so that the gold shall be on top. The goldltaf is Ibeo spread smoothly by blowing over its surface. After the gold has been connected with the circuit, it is covered with glass or a transparent varnish, in order to fix it in place and protect it against accident.
Fig. 102 represents a dry element of this kind that has given very good results, ss represents the selenium between a metallic plate h and a piece of goldleaf i, to which a wire j of the circuit is attached. The current in this case passes from the entire surface of the goldleaf to the lower plate, and traverses all parts of the sheet of selenium. This latter is entirely exposed to the action of the light that traverses the goldleaf. (La Lumiere Electrique.)