Stargazing: Past and Present

by Sir Norman Lockyer

In this way one sees that we have on the barrel an absolute record, by one of the pointers, of the seconds recorded by the clock, and, by the other, of the exact times at which a star has been seen at each wire of the transit instrument.

Now of course what is essential in this method is that there shall be a power of determining not only the precise second or tenth of a second of time, but also the minute at which contact takes place, otherwise there would be a number of seconds dots without knowing to what minute they corresponded; it would be like having a clock with only a second-hand and no minute-hand.

The brass vertical sliding piece shown at the lower left-hand side in Fig. [96], carries at its upper end two brass bars, each of which has, at its right-hand extremity, between the jaws, a slender steel spring for galvanic contact; the lower spring carries a semicircular piece projecting downwards, which a pin on the crutch rod lifts in passing, bringing the springs in contact at each vibration: the contact takes place when the pendulum is vertical, and the acting surfaces of the springs are, one platinum, the other gold; an arrangement that has been supposed to be preferable to making both surfaces of platinum. By means of the screws n and o, which both act on sliders, the contact springs can be adjusted in the vertical and horizontal directions respectively. Other contact springs in connection with the brass bars p and q, on the other side of the back plate, are ordinarily in contact, but the contact is broken at one second in each minute by an arm on the escape-wheel spindle. The combination of these contacts permits the clock to complete a galvanic circuit at fifty-nine of the seconds in each minute, and omit the sixtieth.[^[13]]

In this way we may suppress the sixtieth second, thus leaving a blank that marks the minute; and all that the observer has to do after he has made a record of the transit, is to go quietly to the barrel, and mark the hour and minute in the vacant space. A barrel of this size will contain the observations which would be made in some hours; so that at the end of that time it may be taken off, and it will give, with the least possible chance of error, a permanent record of the work of the astronomer.

It is at once apparent that by the introduction of this application of electricity, astronomy has been an enormous gainer; but so far we have simply given a description of one instrument which has been suggested for that purpose. A few words may be said on other forms.

In the instrument used in the Royal Observatory at Greenwich the rotation of the roller is kept uniform, as we have seen, by a conical pendulum; but there are other methods of attaining this end—there is the fly-wheel and fan, similar to the arrangement for regulating the striking part of a clock; there is the governor used for the steam-engine, and others which give a fairly regular motion—for the motion need not be absolutely uniform, because the dots, which form the points from which to measure, are made by the standard clock.

The particular instant at which each minute occurs may be recorded in another way. The two steel springs above described may be pressed together, not by a pin in the crutch, but by cogs on a wheel attached to the spindle of the escape-wheel of the clock (see Fig. [128]); and then all we have to do to stop the transmission of a current at the sixtieth second is to remove one of the cogs.

Fig. 128.—Wheel of the sidereal clock, and arrangement for making contact at each second.

Another simple method for transmitting seconds’ currents has also been occasionally tried. A wire runs down the whole length of the pendulum, and ends in a projection of such a length that it swings through a small globule of mercury in a cup below it, the pendulum being connected with one wire from the chronograph and the mercury with the other; thus there will be a making and breaking of contact each time the point of the pendulum swings through the mercury. It is uncertain which method is the better; one would prefer that which, under any circumstances, could disturb the pendulum least: but as to which this is opinions differ.