Clarify on Birkhoff's theorem (in universal algebra)

Question

In class we said that, as a consequence of Birkhoff's theorem, the theory of fields is not axiomatizable only by equations. In particular, we saw that the product of two fields in general is just a ring. However I don't understand why if we use a language with $(+,\cdot,-,\frac {1}{}, 1,0)$, where $+$ and $\cdot$ are the binary operations of sum and product, $-$ and $\frac 1 {} $ are the unary operations that give the inverses, and $1,0$ are the two usual constants. It seems to me that in this language the theory of fields is axiomatizable adding to the axioms of a ring (that are all equations) the axiom $\frac 1 x \cdot x=1$, that is an equation too. Probably I'm missing some hypothesis on the language in the Birkhoff's theorem, but I don't see any of them. Thaks for any clarify

Answer 1

The problem is that $(x\mapsto 1/x)$ is only a partial operation in fields (division by $0$ is undefined), that cannot be captured in universal algebra where all operations have to be total.