MEASUREMENTS 



269 



Fio. 157. 

 Plank Counter. 



Marey also used a track, with electric contacts, to obtain a trace 

 of walking, from which he was able to de- 

 duce the speed in terms of the cadence^ 1 ) 



Industry makes great use of re- 

 volution counters for machines, and of 

 tachometers for bicycles and motor vehicles. 

 As an example may be mentioned the meters 

 which are used in weaving mills to indicate 

 the quantity of thread unrolled and in- 

 serted by the shuttle of the weaver, from 

 one selvage to the other, or the length of 

 weft. 



The weft meter is fixed by means of a 

 bracket to the frame at a height at which 



it is easy for the workman to read the figures. He then knows 

 the amount of work produced. 



In the mechanical working of wood, a meter capable of counting 

 the lengths of the planks planed or sawn is used (fig. 157). 



This counter comprises a toothed wheel, A, which is set in 

 motion by the planks. A support, B, fixes the meter to the 

 frame of the machine. The readings are in metres. 



200. The Anemometer. The linear speed of gases is measured 

 by the anemometer, especially when an estimation of the force 

 of an air current is required. In this way, in the mining industry 

 the quantity of air driven into a mine can be ascertained at any 

 given moment. If it is desired to find the speed in a mine 

 gallery, the average value is taken, because this speed varies at 

 different points of the section, being generally greater in the centre 

 than at the wall. 



For speeds of J to 10 metres per second, Combes' classic anemo- 

 meter is sufficient (fig. 158). This 

 instrument consists of a number of 

 flat vanes, mounted at an inclination 

 of 30 on a very thin spindle turning 

 in agate bearings. The vane wheel is 

 exposed to the wind so that the 

 direction of the speed of the latter is 

 FO 138 parallel to the axis oo'. Its motion 



Combes' Anemometer. is communicated to a dial, C grad- 

 uated in revolutions. If N revolutions 

 are recorded in time t, the speed is : 



V = a +m ; 



( x ) Marey, in Trait de Physique Bidlogique (Marey, D'Arsonval, and 

 Gar eel vol. i., p. 187, 1901 Masson, Paris). 



