398 



REPORT — 1892. 



' As a means of calibrating an instrument and preparing the scale the 

 graphic method is very largely used. Several points on the scale through- 

 out the range are carefully noted, and the scale-readings in terms of 

 degrees are plotted in a curve. From this curve the position of standard 

 points — 10, 20, 30, &c. — are read oS. without the trouble of exact adjust- 

 ment of the current or B.M.P. to these values. 



' Tables of corrections for instruments are most conveniently arranged 

 as a curve when they are small. 



' Though these curves are usually plotted by hand from the results of 

 experiment, the instruments may be made to give a continuous record on 



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a moving sheet of squared paper. Voltmeters and ammeters for this 

 purpose are made by MM. Richard Freres, and are in general use in 

 central supply stations, serving as a check on the intermittent readings 

 of the other instruments, and indicating the degree of watchfulness on 

 the part of the attendants, where a constant current or constant difference 

 of potential is required. Mention may also be made of Professor Ewing's 

 magnetic curve-traces presented at the present meeting of the Asso- 

 ciation. 



' It will thus be seen that the use of graphic methods is very general in 

 the practice of electrical engineering, and the range of its applicability is 

 considerable. In fact, in many cases it is the only available means of 

 utilising and drawing deductions from experimental data, and in all 

 cases it is a great help towards a clear comprehension of observational 

 results.' 



Mr. Malcolm Walker, late manager of the ' Sanogen ' Ventilating 

 Company, Liverpool, employs in the regular course of his practice a 

 method of plotting for determining the efficiency of hydro-extractors 

 in which a jet of water is used in order to exhaust the vitiated air from 

 a building. Fig. 27 is a specimen of a curve taken from such an 

 apparatus, and enables the efficiency of the arrangement to be at once 

 determined. 



It will there be seen that the pressures of water used for the jet are 

 used as abscissae, and the quantity of air abstracted as ordinates. In 

 another case the same principle is employed when a fan is used, the 

 number of revolutions forming the abscissae, and the air extracted the 

 corresponding ordinates (fig. 28). This also enables the efficiency of 

 the contrivance to be at once seen, and the proper speed at which the fan 

 should be worked. These observations are made regularly twice a year. 

 Mr. Walker also plots regularly every two months the installation, resist- 

 ance and deterioration of electric surface, fig. 29 being an example of the 

 curves obtained. 



