Described embodiments include apparatus that includes asignal generator and an electric circuit. Thesignalgenerator is configured to supply a signal having both a firstdominant frequency and a second dominant frequency. Theelectric circuit, which includes a reactive component, isconfigured to generate, upon the signal being supplied to theelectric circuit, a magnetic field having both the firstdominant frequency and the second dominant frequency, byvirtue of the reactive component simultaneously resonating atboth the first dominant frequency and the second dominantfrequency. Other embodiments are also described.
