An early type of deflexion anemometer is the swinging-plate instrument, now rarely used. It consists of a vertical plate, suspended on knife edges, which deflects under the action of an air current to such an angle that the restoring torque due to the weight of the plate is equal to the air torque.  The angle of inclination may therefore be taken as a measure of the air speed. In modern types such as the velometer and similar German instruments the plate is replaced by a small vane under spring instead of gravity control, which carries a pointer moving over a scale of air speed. Yet again, the plate may be mounted on a torsion wire passing through an asymmetric vertical or horizontal axis, and there will be a relation between the air speed and the amount of twist of the wire necessary to hold the plate in normal presentation to the current. An anemometer of this kind was designed for low speed by J.P. Rees. It indicated air speeds ranging from 0.05 to 1 m/sec.

一種早期的偏轉(zhuǎn)型風(fēng)速計(jì)是擺動(dòng)片式儀器，現(xiàn)在已很少使用。它是由一塊垂直片懸掛在刀口支承上構(gòu)成的。該片在氣流的作用下發(fā)生偏轉(zhuǎn)，其偏角的大小正好使該片重量產(chǎn)生的恢復(fù)力矩同氣流的力矩相等。因此，這一偏轉(zhuǎn)角即可作為衡量空氣速度的尺度。在現(xiàn)代類(lèi)型的儀器中，例如在速度計(jì)和類(lèi)似的德國(guó)儀表中，這種擺動(dòng)片已用一小葉輪代替，葉輪利用彈簧控制而不用重力控制。葉輪上裝有一指針，可在標(biāo)示有氣流速度的刻度盤(pán)范圍內(nèi)移動(dòng)。再有，可以把擺動(dòng)片裝在一根通過(guò)小片的一條不對(duì)稱(chēng)豎軸或橫軸的扭絲上。在這種情況下，氣流速度與扭絲為使擺動(dòng)片同氣流保持正常位置關(guān)系所要產(chǎn)生的扭轉(zhuǎn)量之間存在某種關(guān)系。J.P.Rees曾經(jīng)設(shè)計(jì)出一臺(tái)這種類(lèi)型的風(fēng)速計(jì)，用來(lái)測(cè)定低氣流速度。它測(cè)定出從0.05~1米/秒范圍內(nèi)的氣流速度。

An example of another type of anemometer in which the airstream displaces an indicator giving an instantaneous reading of velocity or flowrate is the rotameter.  The air flows upwards through a tapered vertical tube in which a float rises until its weight is balanced by the difference of pressure above and below it caused by the motion by the air through the restricted passage around the float.

另一類(lèi)流速計(jì)的典型代表是轉(zhuǎn)子流速計(jì)（或流量計(jì)）。這種儀器的工作原理是氣流使一指針偏轉(zhuǎn)，指示出瞬時(shí)速度值（或流量值）。空氣向上流過(guò)一豎直錐形管，于是管中的浮子（浮球）向上升起，這時(shí)在浮子四周狹窄的通道內(nèi)流過(guò)的空氣在浮子上下形成壓差，當(dāng)浮子的重量與壓差平衡時(shí)，浮子即停止上升。

Electrical or hot-wire anemometers are not extensively used in industrial situations, although certain types have been developed for this purpose. The chief limitation on their use is probably the more elaborate apparatus and manipulation that they require. As laboratory instruments, they may be made to give excellent results; and they are particularly well suited to the measurement of low air speeds, a purpose for which they may be designed to have a considerably more open scale than is possible with pressure-tube anemometers.  The hot-wire anemometer is essential for turbulence measurements, but this field is not discussed in this book.

電風(fēng)速計(jì)也即熱線(xiàn)風(fēng)速計(jì)，在工業(yè)上應(yīng)用不太廣泛，盡管已為此研制過(guò)好幾種。這類(lèi)風(fēng)速計(jì)要求更復(fù)雜的裝置和精心操作，這大概是使它們得不到廣泛應(yīng)用的主要原因。作為實(shí)驗(yàn)室用的測(cè)試儀器，這類(lèi)風(fēng)速計(jì)可制作得很精確。而且，它們待別適合于測(cè)定低氣流速度。為此，可將其設(shè)計(jì)成分度（刻度）比粥壓管型風(fēng)速計(jì)寬得多的形式。熱線(xiàn)風(fēng)速計(jì)對(duì)湍流測(cè)量是必不可少的。但這一領(lǐng)域的問(wèn)題本書(shū)不予討論。