740.—The author has made many chains of 500 links; in Fig. 344 a part of one is shown full size. This band, as we may term it, is wound upon a reel in an iron case, Fig. 343. A spring brake is placed at the position A, which holds the reel and prevents the band from springing out into loose hoops when it is run out. The 50 and 100 links are indicated by short lengths of brass tube placed over the band—single at the 50 links, but numerically indicated by number of bands as 2, 3, and 4 chains. In Fig. 344 a 50 and a 300 links are shown; weight, 3½ lbs. This flat, narrow, steel band chain was unknown until introduced to the notice of the profession in the first edition, 1890. It is now in very general use, and lengths may be had from stock of 2, 3, 4, or 5 chains, or 200, 300, or 400 feet wound upon a steel cross.
Fig. 345.—Richmond's tension handle.
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741.—Richmond's Tension Handle.—Various devices have been employed for giving equal tension to chains and bands to ensure equality of measurements. Salter's spring balance has been very commonly used attached to one handle of the chain to give a uniform pull, say of 15 lbs. This appears to answer very well. Mr. Richmond, surveyor, of Sydney, has devised a very simple plan for tension of light bands, which, being lighter and attached, is much more convenient than Salter's balance. This is shown Fig. 345. The band passes through a fitting in the centre of the handle, and a spiral spring is fixed to this and the band at a short distance along it. By pulling the handle a given tension can be applied, which is shown by the mark it reaches towards the end of the band. This is adjusted to standard length, and a small notch is placed in the centre of the end, from which a plummet may be suspended if necessary.[54]
The engraving is of a slightly modified form by the author, in which a thin tubular cap covers the free end of the band to save this exposed part from accidents.
Fig. 346.—Copper case thermometer for suspending to a band chain.
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742.—Chain and Band Thermometer.—Where very great accuracy of chain or band measurement is aimed at, temperature is taken to allow for expansion of the metal. A thin plain glass thermometer of the clinical form is the most sensitive of any. This is carried in a wooden pull-off case lined with indiarubber. When it is used it is placed upon the ground by the side of the chain. The delicacy of the clinical form of thermometer is often objected to by the practical surveyor, hence there are several other forms with boxwood and ivory scales. These are not very satisfactory, as the boxwood and ivory retain the heat of the body, from being carried in the pocket, for a long time after exposure. The author has enclosed the clinical form of thermometer in a copper case with open face, Fig. 346. The copper being a good conductor of heat, this is very sensitive to the temperature of the air. Two turn-down hooks are placed at the ends of the tube to suspend it on the band. The thermometer stem has two indiarubber caps, so that it will bear dropping on grass. It is contained in the same form of pull-off case as the clinical.