MEASURES, WEIGHTS

MEASURES, WEIGHTS, &c. USED IN THIS WORK.

Temperature is measured by degrees of Fahrenheit's scale, of which the freezing point is

32, and the boiling point 212.

Heat is commonly measured by the number of degrees it would increase the temperature of

a given quantity of water at 60, with the barometer at 30 inches. 1

Mechanical power is measured by the elementary horse power, as settled by Watt. A

horse power is = 33,000 ffis. raised one foot high per minute, or = 550 Ibs. raised one

foot high per second ; and a day's work of a horse is this power acting 8 hours.

This horse power is, in French measures, 4661 kilogrammes raised one metre high per

minute.

The pound is the avoirdupois pound, = 7000 troy grains, = '4535 French kilogrammes.

The foot is = -3048 French metre.

( 30 inches 1 , An atmosphere is =

| .^62 metre j

of mercury.

= 14*70 Tbs. per square inch.

= 11 '55 circular

1 It is more properly measured by the volume of water at 60, which it would raise exactly one degree ; this measurement

being strictly proportional to the absolute quantity of heat. ED.

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M❤K

BASIC KNOWLEDGE OF ELECTRICITY

BASIC KNOWLEDGE OF ELECTRICITY

Electric Circuit-a network of paths designed for electric current to carry out a

specified function.

Circuit Breaker - one form of switch providing ON/OFF function and

automatic tripping of the electric circuit in the event of short

circuit or circuit overload. The point at which the breaker

trips depends on the rating of the circuit. The breaker can be

reset to operate again after the fault has been cleared.

Fuse - a conductor, usually in the form of a wire or a cartridge, of low melting

point material placed in the electrical supply circuit and forms part of

the circuit. It serves to protect the electric circuit from short circuiting

or overloading. It melts to interrupt the electricity supply because the

excess current produces a large amount of heat. Unlike the circuit

b r e a k e r, it must be replaced with a new unit after blowing.

Earth Line - a conductor or wire which electrically connects the boiler metal

casing with the earth terminal provided by the electricity supply

c o m p a n y. This line plays a very vital role in the prevention of

electric shock.

Electric Shock - this is a phenomenon whereby the human body has become a

path for electric current. The human body may suffer burns

and heart difficulties which can result in death.

When an electric wire of high potential becomes loose or exposed and is in

contact with the metal casing of the boiler, the casing will also be at a high

potential relative to the earth. If the earth line is also defective, the casing will

be maintained at a high potential. There may not be any indication of the high

potential but when a person is in contact with the casing and he is also in

contact with the earth, an electric path for current is established through the

human body. Potential differences above 50V, A.C. or D.C., are able to drive

electric current through the body. This is what is termed an electric shock.

Alternating current has a more serious effect in that it can cause the victim to

become paralysed while still holding firmly onto the faulty electric part. The

normal mains supply in Hong Kong is nominally 220V A.C. which is well

above the threshold voltage of 50V. However, it should be noted that,

depending on the physical conditions of a person, potential difference less than

this threshold voltage has been known to cause serious injuries.

I M P O RTANT NOTE:- Never try to repair any faulty electric circuit. The

repair should be carried out by a registered.

electrical worker only.

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M❤K