A text-book of physics : with sections on the application of physics to physiology and medicine / by R.A. Lehfeldt.

  • Lehfeldt, R. A. (Robert Alfred), 1868-1927.
Date:
[1902]
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of physics : with sections on the application of physics to physiology and medicine / by R.A. Lehfeldt. Source: Wellcome Collection.

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    For measuring large and small lengths, the following units are in use :— Kilometre = io5 cm.1 Metre Millimetre Micron (written n) Micromillimetre (written /*/*) = io1 cm. = io-1 cm. = io-1 cm. = io-7 cm. For a description of the numerous instruments for measuring lengths large and small, for constructing scales, and for com¬ paring scales with one another, reference must he made to the works on practical physics. From the unit of length are derived in a systematic manner the units of area and volume. The unit of area is the square centimetre (sq. cm.); that of volume the cubic centimetre (c. c.). A litre is 1000 cubic centimetres. The scientific unit of time is the second. This, unlike the centimetre, is defined by reference to a natural phenomenon, being -8-<; ] 00 part of the mean solar day. It is often necessary, in physical (and physiological) experi¬ ments, to measure, and, if possible, to record automatically, periods of time too short for observation in the usual manner by means of a clock. An excellent standard for short intervals of time is a tuning-forJc whose vibrations are maintained electro- magnetically. A short bar of iron, wound with a coil of insulated wire a, constituting an electromagnet (Fig. i), is fixed between the prongs of the fork : a short piece of platinum wire is attached to one prong in such a manner, that when the prong is pressed 1 Very large and very small numbers are commonly expressed by means of a power of io. Tims 5,870.000,000 may be written 5 87 x ro°, and 0-0000026 as 2-6 x io-6. The rule is, move the decimal point as many places to the right as the index shows if it be positive, to the left if negative.
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