In Separate containers, rinse the Urad dhal and rice.  Discard the water.  Do not mix together.  

Place fresh water in each container about 4 inches over the Urad dhal and rice.  Soak 6-8 hours or overnight.

Drain the water completely from each container.  In a blender, blend the Urad dhal with 1 cup of water.  Blend to a smooth paste.  Remove the paste from the blender and set aside in a container.

Next, place the rice in the blender with ½ cup water.  Blend to a smooth paste.

Add the rice paste to the Urad mixture.  Mix together with a spoon.  You should get a smooth batter with a pouring consistency.  It should have the consistency of a pancake batter.  

In 1 cup of batter add ⅛ tsp baking soda, ⅛ tsp lime juice and salt to taste.

Mix well and allow to rest for 30 min.

Heat a nonstick pan on medium high.  When pan is hot, flick a few drops of oil into the pan and spread it around with a spatula. Remove the pan from the heat and pour a ¼ cup of batter into the middle of the pan and then spread it around with a ladle so that the batter spreads out into a thin circle.  (It is like making a crepe). 

Place pan back on the stove and cook on medium high heat for about a minute or a little longer.  Add a few drops of oil on the pan around the dosa and then sprinkle a few more drops on the dosa itself.  When the edges of the dosa start to turn light brown, gently loosen it with the spatula and flip.  Cook for about another minute or a little longer.  Remove dosa from pan.

Before making the 2^nd dosa, remove the pan from the heat for about 15-30 seconds to cool the pan slightly then pour ¼ cup of batter and spread out into a large circle.  Place the pan back on the stove and repeat the process until you use up all of your batter.  

The batter does not have to be used all at once.  It can be stored in an airtight container in the refrigerator for up to 1 week. 

Dosa is better when served hot off the stove.