The Pavlov experiment Thomas (1994) cites is operant rather than Pavlovian, hence Pavolv still never used a bell as a conditioned stimulus (CS)
2. The bell in question does not conform to the popular image of a hand-held metal object that a school master might ring. This is an electric bell.
3. Regardless of its nature, the bell seems actually to have been more of a discriminative stimulus than a conventional conditioned stimulus (CS). Although there may well have been a salivary controlled response (CR) conditioned to the bell, it presumably went unmeasured. In its place, Pavlov reported the operant response of running to a desired location to receive a food reward. This fits the definition of a discriminated operant procedure.
4. The point of this procedural hair-splitting is that it allows us to assert that Pavlov never used a bell as a CS.
Littman, R. (1994) Bekhterev and Watson Rang Pavlov's Bell: A Reply to Catania's Query. PSYCOLOQUY 5(49) pavlov-bell.1.littman.
Pawlow, I.P. (1923). New researches on conditioned reflexes. Science, 58, 359-361.
Thomas, R.K. (1994) Pavlov Used a Bell: Commentary on Littman on Pavlov-Bell. PSYCOLOQUY 5(63) pavlov-bell.2.thomas.