Power-supply transformers have leakage inductance and parasitic capacitance. Mechanical resonators are analogous to electrical LC circuits consisting of inductance and capacitance. The current and voltage reverse direction, forming a harmonic oscillator between the inductance and capacitance. These can generate an impedance equivalent to a negative inductance or capacitance. Good high-frequency response requires carefully designed and implemented without excessive leakage inductance or stray capacitance. Loudspeakers present a reactive load to an amplifier (capacitance, inductance and resistance). This allowed electrical engineers to simplify calculations for inductance and capacitance in power transmission lines. An electronic component designed to add inductance to a circuit is called an inductor. Some inductors have an adjustable core, which enables changing of the inductance. Thus, the choke presents little inductance or impedance to DM currents. He was also one of the first to report negative capacitance and inductance. The inductive parts of a capacitor are summarized in the equivalent series inductance, or ESL. All current excites current and induces secondary voltage which is proportional to the mutual inductance. Similarly to ESR, the capacitor's leads add "equivalent series inductance" or ESL to the component. The conductors and leads introduce undesired inductance and resistance.