Final answer:
The conjunction 'Not only... but also' in the sentence is a correlative conjunction, linking two equally important ideas within the sentence.
Step-by-step explanation:
The conjunction in the sentence "Not only could the camera perform two tasks, but the box was much lighter than its rival, the kinetoscope" is correlative. Correlative conjunctions work in pairs to join together words, phrases, or clauses that are usually of equal importance. In this case, the correlative conjunctions used are "not only" and "but also," which help to correlate or link the two ideas presented in the sentence: the camera's functionality and its weight comparison to the kinetoscope.