Scale Faithful is the name given to the pointer present on older scales to indicate the heavier side. Nowadays, the expression is popularly used to qualify something like decisive or reliable.
The expression also identifies something or someone as being of crucial importance for a given result.
Examples:
"You will be the tip of the balance in this mission."
"The PT caucus, for being numerous, will be the tip of the balance in this vote."
In courts, the expression is often used to refer to a decisive vote or position in a judgment.
Examples:
"Rosa Weber will be the tip of the balance in the Lula case."
"In cases of a tie, the president of the court will be the tip of the balance."
Scale faithful is also used to designate something you can trust.
Examples:
Neymar is the balance of the Brazilian team.
The mainstay of my company's balance is service.
Origin
The origin of the expression "faithful of the scale" dates back to the time when scales indicated the weight by comparison. In one of the dishes, the desired weight was placed and in the other, the product to be purchased was placed. The pointer in the middle of the scale (called the faithful) showed whether or not there was balance between the plates and, based on it, the price was decided.