Chapter XIV

FORMULÆ

In the construction of electrical apparatus there are many things, such as paint, cement, non-conducting compounds, and acid-proof substances, that are necessary in assembling the parts which make up complete working outfits. Accurate formulas and directions for these things will save the amateur trouble and expense, since they indicate the materials which have been put to the test of time and wear by others who have had abundant experience along these lines.

The amateur will not need a large number of compounds, but such as are necessary should be of the best. Those which are described in this chapter can be relied upon to give working results.

Acid-proof Cements

One of the best acid-proof cements is made by adding shellac, dissolved in grain alcohol, to red-lead until it is at the right consistency. It can be used in liquid form or in a putty-like paste. The consistency is governed by the amount of shellac added to the red-lead. The lead should be pulverized and free from lumps. This cement can be mixed in a small tin cup or on a piece of glass, with a knife having a thin blade.

It should be used as soon as it is mixed, since it “sets” as quickly as shellac, and then dries from the outside towards the middle. In a week or two it will become dry and hard like stone.

Another cement, which will also dry as hard as a stone and will hold soapstone slabs together as if they were of one solid piece, is made of litharge (yellow lead) and glycerine. The glycerine is added to the pulverized litharge to make a paste, or it can be mixed and kneaded like thin putty. It should be used very soon after mixing, as it sets rapidly.

Hard Cement

A medium hard cement is made from plaster of Paris, six parts; silex, or fine sand, two parts; dextrine, two parts (by measure). Mix with water until soft; then work with a trowel or knife.

Soft Cement

A good soft cement is made of plaster of Paris, five parts; pulverized asbestos, five parts (by weight). Add water enough to make a soft paste, and use with a trowel or knife. This is a heat-proof compound and is commonly known as asbestos cement.

Very Hard Cement

One of the hardest cements that can be made is composed of hydraulic cement (Portland or Edison), five parts; silex, or white sand, five parts (by measure). Mix with water and use like plaster with a trowel or spatula.

Care must be taken when the parts are combined to see that the cement is free from lumps. These must be broken before the silex, or sand, and water are added. Then the two parts should be mixed together at first and moistened afterwards. The proper way is to place some water at the bottom of a pan; then add the dry mixture by the handfuls, sprinkling it around so that the water can enter into it without forming lumps. Keep adding and mixing until the mass is at the right consistency to work.

All these cements are acid-proof.

Clark’s Compound

For exterior insulation, where the parts are exposed to the weather, a superior compound is made up of mineral pitch, ten parts; silica, six parts; tar, one part (all parts by weight). This is called Clark’s compound, after the man who invented it and used it successfully.

It is heated, thoroughly mixed, and used with a brush or spatula.

Battery Fluid

A depolarizing solution for use in zinc-carbon batteries like the Grenet is composed as follows:

Dissolve one pound of bichromate potash or soda in ten pounds of water (by weight). When it is thoroughly dissolved add two and one-half pounds of sulphuric acid, slowly pouring it into the bichromate solution and stirring it with a glass rod. The addition of the acid will heat the solution. Do not use it until it has entirely cooled.

Glass Rubbing

To rub the edges of glass, such as the disks for Wimshurst machines, obtain a piece of hard sandstone, such as is used for sharpening knives or scythes. The glass is placed on a table so that the edge extends beyond. Oil of turpentine is rubbed or dropped on the surface of the stone, and the edge of the glass is moistened with a rag soaked in the turpentine. Hold the glass down securely with one hand, and with the other grasp the stone and give it a forward and backward motion, grinding the glass along its edge and not crosswise. With care and patience a rough edge can soon be brought to a smooth one, and a soft, rounded corner substituted for the hard, angular, cutting edge that makes the glass a difficult thing to handle. Use plenty of lubricant in the form of oil of turpentine to make the work easy.

Acetic Glue

One of the best glues for glass and wood or glass and fibre is made by placing some high-grade glue (either flake or granulated) in a cup or tin and covering it with cold water. Allow it to stand several hours until the glue absorbs all the water it will and becomes soft; then pour the water off, and add glacial acetic acid to cover the glue. The proportion should be eighteen parts of glue to two of acid. Heat the mass until it is reduced to liquid, stirring it until it is thoroughly mixed. When ready for use it should be poured into a bottle and well corked to keep the air away from it.

Insulators

Apart from glass and porcelain, insulators can be made from non-conducting compounds, the best of which is molded mica. Ground mica or mica dust is mixed with thick shellac until it is in a putty-like state. It may then be forced into oiled molds of any desired shape. Hydraulic pressure is employed for almost every form of molded mica that is made for commercial purposes; but as a boy cannot employ that means to get the best results, he must use all the pressure that his hands and a flat board will give.

Another compound is made from pulverized asbestos and shellac, with a small portion of ground or pulverized mica added, in the proportion of asbestos, six parts; mica, four parts. Shellac is added to make a pasty mass, which is kneaded into a thick putty and forced into oiled molds until it sets. It is then removed and allowed to dry in the open air, and the mold used for more insulators.

Non-conductors

When working in different materials that seem adapted to electrical apparatus, it is necessary to know whether they can be used safely or not. Often a material seems to be just the thing, but if it should be a partial conductor, when a non-conductor is desired, it would be hazardous to use it. A list of non-conductors is therefore valuable to the amateur. Some of the principal non-conductors, among the many, are as follows: glass, porcelain, slate, marble, hard stone, soapstone, concrete (dry), hard rubber, soft rubber, composition fibre, mica, asbestos, pitch, tar, shellac, cotton, silk; cotton, silk and woollen fabrics, transite (dry), electrobestus (dry), duranoid; celluloid, dry wood (well seasoned), paper, pith, leather, and oil.

While this account of formulæ and material might be extended, this is not necessary inasmuch as the formulæ and practical directions which have been given will answer all usual practical requirements.

Insulating Varnish

There are several good insulating varnishes that can be used in electrical work, the most valuable being shellac dissolved in alcohol and applied with a brush. To make good shellac, purchase the orange-colored flake shellac by the pound from a paint-store, place some of it in a wide-necked bottle, and cover it with alcohol; then cork the bottle and let it stand for a few hours. Shake the bottle occasionally until the shellac is thoroughly dissolved. It can be thinned by adding alcohol. Always keep the bottle corked, taking out only what is necessary from time to time.

Another varnish can be made by dissolving red sealing-wax in alcohol and adding a small portion of shellac. This can be applied with a soft brush, and is a good varnish. When colors are to be applied to distinguish the poles, red is used for the positive current-poles and blue or black for the negative, if they are colored at all.

A very good black varnish is made by adding lampblack to shellac; another consists of thick asphaltum or asphaltum varnish. This is waterproof, and dries hard, yet with an elastic finish.

Battery Wax

For the upper edges of glass cells, such as the Leclanché or other open-circuit batteries, there is nothing superior to hot paraffine brushed about the upper edge to prevent the sal-ammoniac or other fluids from creeping up over the top. The paraffine can be colored with red-lead, green dust, or powders of various colors if desired, but generally the paraffine is used without color, so that it has a frosted-glass appearance when it is cool and dry.

A black wax for use in stopping the tops of dry cells and coating the tops of carbons is composed of paraffine, eight parts; pitch, one part; lampblack, one part. Heat the mixture and stir it until thoroughly mixed; then apply with a brush, or dip the parts into the warm fluid.

Another good black wax is composed of tar and pitch in equal parts. They are made into a pasty mass with turpentine heated over a stove, but not over an open flame, because the ingredients are inflammable. The compound should be like very thick molasses, and can be worked with an old table-knife.