Question

when is zero error in callipers negative?

Answer 1

because error is always taken as negative either it is coming more than 0 or less than zero

Answer 2

a negative zero error is when the zero mark on the vernier scale lies before the zero on the main scale when the caliper is closed. In other words the reading is less than zero. Negative zero errors can be taken care of by adding the value of the error to the total reading. For example is the total reading is 3.56 cm and the zero error is -0.05 then the actual reading will be 3.61 cm.