The following measurement has only 3 significant figures. Please explain why?
.000305

Number of Significant Figures: 3

The Significant Figures are 3 0 5

This is because the three zeros are considered trailing zeros because they're not between two numbers or after. Zeros attached to the beginning of a number in this way are called leading zeros. In decimals, this idea of zeros that don't add value to a number can be extended to trailing zeros. A trailing zero is any zero that appears to the right of both the decimal point and every digit other than zero.

Explanation:

Trailing zeros (zeros after non-zero numbers) in a number without a decimal are generally not significant (see below for more details). For example, 400 has only one significant figure (4). The trailing zeros do not count as significant. Trailing zeros in a number containing a decimal point are significant.

the amount of current in the coil

no. dot product   =0 when vecs at right angles