| Millimetre | = | 0·039 | inches | 1 mill | = | ·0254 | millimetres. |
| Centimetre | = | 0·393 | ” | 1 inch | = | 2·5399 | centimetres. |
| Decimetre | = | 3·93 | ” | 1 foot | = | 3·3480 | decimetres. |
| Metre | = | 39·37 | ” | 1 yard | = | ·91439 | metres. |
| Cubic metre | = | 35·32 cubic feet or 1·31 cubic yards. | |||||
Electrical Measurements.
The Paris Congress Units (1884) are now universally adopted and consist as follows:
Electro-motive Force, and Potential (E).—The Volt. The legal volt is ·926 of the E. M. F. of a Daniell’s cell, which for rough purposes may be taken as a volt.
Resistance (R).—The Ohm. The legal ohm is now represented by the resistance of a column of mercury of a square millimetre in section at the temperature of zero centigrade 1·062 metres long.
Current (C).—The Ampère. This is the strength of current sent through a wire having the resistance of 1 ohm at the E. M. F. of 1 volt.
Quantity (Q).—The Coulomb. It is the quantity of electricity given by an ampère in a second. One coulomb decomposes ·00142 grain of water.
Heat or Work (W).—The Joule, or Volt-Coulomb, is the work done by 1 coulomb in 1 ohm. The work done by any current per second is obtained in ergs by the product of the current into the electro-motive force producing it or W = CE or W = C²R. The Erg is the C. G. S. unit of work.