This solid layer of the earth is made of mostly iron and nickel.

Answer

Explanation:

Yes, it's true that the solid layer of the earth is known as the most dense part as it is made up of the heavy metals like iron and nickel. Inner part is the hotter part due to the high pressure and temperature. It has the temperature of about 5,200°C and the pressure of 3.6 million atm but still the iron and nickel are present there in the solid form as they withstand such high temperature and pressure values.

need to see circuit to get time constant

an electric circuit