Sentence:  In the "Hotchkiss superdip" the total intensity is determined from the deflection of a system consisting of a magnetic needle to which is fastened a <WSD>counterarm</WSD> carrying a small weight and making an adjustable angle with the needle. 
Meaning of the word marked with <WSD> tags:  A weighted arm, or an arm that holds a weight, which acts as a counterbalance.