The Meaning of Relativity / Four lectures delivered at Princeton University, May, 1921

by Albert Einstein

. The vector character of the second term on the right is obvious. In order that the first term on the right may also be a vector it is necessary for

to be a tensor. Then by differentiation and contraction

results, and is therefore a vector, as it also is after multiplication by the reciprocal scalar

. That

is a tensor, and therefore transforms according to the equation