Turning 'What value of T is just small enough to make the bulb light?' into its affirmative form

Question

I am somehow confused with regard to the way the following question is constructed

What value of T is just small enough to make the bulb light?

The question below, in contrast, is clear and we can see how it was formed starting from its 'original' form.

What is the value of T that is just small enough to make the bulb light?

First: The affirmative form

The value of T that is just small enough to make the bulb light is (What).

Second: Bringing the wh-word to the front

(What) The value of T that is just small enough to make the bulb light is.

Finally: Subject-verb inversion

What is The value of T that is just small enough to make the bulb light?

I cannot apply the same procedure to first question to see how it was obtained; is it even grammatical?

Answer 1

While one can think of a question as being derived from a statement, a native sp3aker does not typically first construct a statement and then transform it into a question. Similarly, while a question like your original example:

What value of T is just small enough to make the bulb light?

can be thought of as being an elided form, constructions of the general form

What X is Y

or

What value of X is enough to Y

are grammatically valid as they stand.

Here "T" is some property that has a numerical value. If T is small enough, a bulb lights up. This obviously assumes some particular equipment, but that is not relevant to the grammer. The point is that T has a value, and the question is: "What value of T has {Effect Y}?" A parallel question might be

What color traffic light causes drivers to stop?

(The answer, of course, is "Red".)

That question is fine as it stands, and does not need to be recast as:

What color of traffic light is it that causes drivers to stop?

The basic pattern here is "What X is Y?", not "What X is it that is Y?"