MS

There are 360 electricaldegrees in a cycle; 180 degrees in a half-cycle. The number of degrees from the beginning of the cycle until

the SCR is gated ON is referred to as the firing angle, and the number of degrees that the SCR remains conducting is known as the conduction angle.

In SCR Phase Control, the firing angle, or point during the half-cycle at which the SCR is triggered, determines the amount of current which flows through the device. It acts as a high-speed switch which is open for the first part of the cycle, and then closes to allow power flow after the trigger pulse is applied.so it is used.

Procedure
- Place 50 g sample of cement on a clean and dry # 100 (hole size of 0.15 mm) sieve, with the pan attached to it. While holding the sieve and the uncovered pan in both hands, sieve with a gentle wrist motion until most of the fine materials have passed through and the residue looks fairly clean.
- Place the cover on the sieve and remove the pan. Now holding them firmly in one hand, gently tap the side of the sieve. The dust adhering to the sieve will thus be dislodged and the underside of the sieve may now be swept clean.
- Empty the pan and thoroughly wipe it out with cloth or a napkin. Replace the sieve in the pan and carefully remove the cover. Return any coarser material in the cover, caught during tapping of the sieve.
- Continue the sieving as described earlier, without the cover, depending on the condition of cement.
- Continuously rotate the sieve along with gentle wrist motion, taking care not to spill any cement. Continue this for about 9 minutes. Replace the cover and clean, following the same procedure as described earlier. If the cement is in proper condition, there should now be no appreciable dust remaining in the residue or adhering to the sieve and the pan.
- Conduct 1-minute test as follows: Hold the sieve in one hand, with the pan and the cover attached, in slightly inclined position and move it backward and forward in the plane of inclination. At the same time gently strike the side about 150 times per minute against the palm of the other hand on the upstroke. Perform the sieving over a sheet of white paper. Return any material escaping from the sieve or pan and collecting on the paper.
- Weigh the amount of cement remained.
W =
Rt =
Calculation
F = 100 – (Rt/W) 100 =
Where F = fineness of cement expressed as the percentage passing # 100 sieve, Rt = weight remaining in # 100 sieve, and W = total weight of the sample in grams.

CH

5- SEMICONDUCTERS (FROM LED)

6- OPTICAL PROPERTIES OF MATERIAL

8-OTHER ENGINEERING MATERIAL

Defects in timber reduce its strength and affects its appearance

DEFECTS ARE:-                         

Bow: is curvature flatwise from a straight line from one end of the board to the other.

Burrs: abnormal growth caused by a mass of small twigs that become trapped in the grain as the tree grows.

Checks: surface splits caused by the timber rapidly drying out

Cup: the entire length of the plank curves along its grain.

Dry rot:   dry rot is a condition in which the wood decays and crumbles into small granular particles. it is caused by a fungus

Pitch: heavy concentration of resin in wood cells in a single location

Reaction Wood: timber subjected to constant winds or some other force which causes the distortion of timber will form reaction wood

Shakes:during growth cause fibre separation within the trunk resulting in shakes

Stains: are discolourations of the board which damage the natural colour but do not affect the strength.

Twist: A spiral distortion along the length of a piece of timber.

Want: The absence of wood, other than wane, from the arris or surface of a piece of timber.

Warp: variation from a true and plane surface. It includes bow, cup and twist and is often caused
by irregular seasoning.