An arc welding $$DC$$ power source has a linear power source characteristic with open circuit voltage $${V_O} = \,\,80$$ volts and short circuit current $${{\rm I}_S} = 1000amps.$$ The voltage-length characteristic of the arc is given by $$V=20+4L$$ volts where $$L$$ is the arc length in $$mm,$$ calculate the optimum length of arc for obtaining maximum arc power at welding. What voltage and current setting should be done on the power source for maximum arc power.

Two cubical castings of the same metal and sizes of $$2cm$$ side and $$4cm$$ side are moulded in green sand. If the smaller casting solidifies in $$2mins,$$ the expected time of solidifications of larger casting will be

Increase in water content in moding sand causes

Chills are used in molds to