A stethoscope head with adjustable audio frequency includes a head body, a diaphragm and a fastener. Furthermore, cleaning compositions comprising hydroxyethyl cellulose, diethylene glycol monoethyl ether, tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, sodium alkyl ethoxy sulphate, sodium lauryl sulphate, cocoamide diethanolamine, a sodium di-alkyl sulphosuccinate, and water which demonstrates superior viscosity stability are disclosed. A control module applies a voltage potential to the electrode and detects a combustion ionization signal and determines if there is an oscillation in the combustion ionization signal indicative of the occurrence of combustion stability or the onset of combustion instability. A wide range of financial products are supported, particularly investments and insurance. A method of making a drill bit comprises investment casting a shank and a cutting head of a first material. The composition, and the cured product thereof, is useful in optical devices such as charged coupled devices, light emitting diodes, lightguides, optical cameras, photo-couplers, and waveguides.