Electricity.

Date:
[1829]
Catalogue details

Licence: Public Domain Mark

Credit: Electricity. Source: Wellcome Collection.

    16/150 (page 26)
    amalgam. Take now a piece of leather about five or six inches square, and spread over one side of it a quantity of amalgam ; throw back the silk flap, and, turning the machine gently round, apply the amalgam side of the leather to the cylinder for the space of two minutes or more, as circumstances may require, during which time the excitation will be observed to increase ra¬ pidly. The cylinder must next be wiped per¬ fectly clean with an old silk handkerchief, and afterwards with a soft dry linen cloth. Let the cushion be again removed; and the amalgam which appears above and below the line of con¬ tact with the cylinder carefully scraped off, the silk flap wiped with a linen cloth, and the whole returned to its place and made fast. If now the cylinder be turned slowly round, streams of the electric fluid will be seen rushing from the silk flap round the lower part of the cylinder, at¬ tended with a hissing and snapping noise, while large brushes of the same, of several inches in length, may be observed flying off from the lower edge of the silk into the surrounding air. The machine is now fit for use, and may be fastened to the table, after which the whole of its parts are to be well wiped with a warm and dry linen cloth to free them from dust. 85. The operator, however, must not expect this high and rich state of excitation to be of long duration. The cylinder will soon cool; dust will be attracted by the action of the machine ; and the moisture produced in the air of the room by the breath of his audience, will, by their united effects, render all his efforts to produce a copious supply of electricity entirely fruitless. 86. To- remedy this defect, which gentlemen who deliver public lectures on electricity have often found to be a grievous one, provide a box of thin plate iron, ten or twelve inches long, four inches wide, and one inch and a half in depth, with a lid to fit very easily over it. In this box a piece of bar iron of about six inches in length, three in breadth, and half an inch in thickness, after being heated in the fire to a dull red heat is to be placed, the lid of the box put on, and the whole, on a suitable iron stand, placed under the cylinder, on the board of the machine in a longi¬ tudinal direction. The radiation of heat from the iron will effectually preserve the equality of the temperature of the surrounding air for a consi¬ derable length of time, and indeed for any length of time required, since, by employing two bars of iron, the one may be kept in the fire while the other is in the box, and thus no other interrup¬ tion in the course of the experiments will be ne¬ cessary beyond what is occasioned by the chang¬ ing of the irons. By this means the machine may be made to act in full vigor under the most dis¬ advantageous circumstances. 87. Dr. Brewster mentions a method used by Mr. Ronalds of aiding the excitation of the ma¬ chine, which he represents as being attended with the greatest advantages. He thus describes it: In the cylinder machine, the rubber is placed in front of a half cylinder of copper, which commu¬ nicates with a copper pipe, that serves for the support of the rubber. A very small spirit-lamp, whose burner consists of only one thread of cot- ion, is placed immediately beneath the mouth or lower end of the copper pipe, so as to keep the rubber and the parts adjacent to it always hot and dry. Besides this contrivance, Mr. Ronalds places his prime conductor upon a glass support, so that a similar spirit-lamp may be placed below it in order to convey heat to its interior. Mr. Ronalds remarks, that a cylinder machine thus constructed, of half the dimensions of one made upon the usual plan, is highly and perma¬ nently effective. The same principle may easily be applied to plate machines. The heat is sup¬ posed to assist the excitement by promoting the oxidation of the amalgam. 88. This method may certainly, in some de¬ gree, prove beneficial; but it will be found to be in many respects inferior to the simple method above mentioned. In the first place, the radia¬ tion of heat will be neither so general nor so great; in the second place, the flameof a lamp or a candle absorbs the fluid ; and in the third place, the light, which must necessarily be emitted from two spirit-lamps, would prove highly detrimental to the effect of those experiments which require to be performed in darkness. To which may be added, the expense of the lamps, and the spirit to be consumed. 89. Where a plate machine of large dimen¬ sions is used, this additional article will not be required; for the great thickness of the glass renders it capable of retaining the heat much longer than can be done by a thin cylinder; for which reason it must be obvious that, if cylin¬ ders were made much stronger than they gene¬ rally are, their action would be effectual for a greater length of time than it is. Opinion, how¬ ever, runs in favor of thin cylinders, but the consistency of such opinion remains to be shown. It is well known that the old globular machines, which were made of thick glass, when once put in a state of powerful excitation, retain that state much longer than the modern thin cylinder will do under the same circumstances. 90. The following directions for preparing the amalgam for electrical machines are given by Mr. Singer, in his treatise, already mentioned. Melt in an iron ladle two ounces of zinc with one ounce of tin, and, while this mixture is in a fluid state, pour into it six ounces of mercury ; let the whole be then put into an iron or wooden box, and agitated until it be quite cold. It must then be reduced to fine powder in a mortar, and mixed with sweet hogs’ lard to the consistence of thick paste. This part of the process need not.be performed till the amalgam is wanted for use. This amalgam, he remarks, answers ex¬ ceedingly well, but, he afterwards adds, I have since made it with a still less proportion of mercury with equal effect. The proportions may be two ounces of tin, four ounces of zinc and seven ounces of mercury. The mercury must be heated to about 300° Fahrenheit, before the fused metals are added to it. When thy amalgam has been agitated until cool, and finely powdered, it is to be mixed with hogs’-lard be trituration in a mortar; and should it at any time become hard, more lard must be added and the trituration be repeated. 91. The application of the electrical apparatus to the purpose of experiment will afford the best